PoE 3.9 Shadow CoC Assassin Endgame Build

Let's see how this PoE 3.9 Assassin CoC build works. Ice Nova synergises with Frostbolt, being able to expand from up to 2 Frostbolts, basically dealing twice the damage per cast. With enough cooldown recovery speed and attack speed, we can get up to 7.5 casts per second for each spell. We also convert all damage to fire using Cold to Fire gems and Avatar of Fire.

Quick Jump

Pros & Cons

Skill Tree

Bandits

Help Alira. Everything she gives us is useful. Kill all can also be an option if you prefer to.

Ascendancy 

For leveling, my preferred order for ascendancy notables is Opportunistic > Ambush and Assassinate. After Merciless lab, remove Ambush and Assassinate and pick Unstable Infusion and Deadly Infusion. For Uber lab, pick Mistwalker.

Below, there's an explanation about our choices.

Pantheon

Major: Lunaris

Minor: Any one will work well, depending on your preference. I'd recommend Garukhan for general use, and Ryslatha for lab (as we have no/low base health regen).

Passive Skill Tree & PoB

https://pastebin.com/h5FpsCcc

Gems Setup

Cospri 1: Frostbolt setup

Frostbolt - Cold to Fire Support - Combustion Support

Frostbolt shall be placed on the mainhand Cospri. Cold to Fire is the best support for us. Combustion reduces enemies fire resistances, benefitting all of our damage gems.

Cospri 2: Ice Nova setup

Ice Nova - Cold to Fire Support - Elemental Focus Support

Ice Nova shall be placed on the offhand Cospri. Concentrated Effect is slightly better than Elemental Focus, but the reduced AOE from it (and the already too high damage) made me keep Elemental Focus. It can be better for bosses, though.

Chest: COC setup

Cyclone - Wave of Conviction - Increased Critical Strikes - Faster Attacks - Fortify - Cast On Critical Strike

Differently from the original version, in which we had two Ice Nova gems, here we have a third spell on our 6link: Wave of Conviction. As we only deal fire damage, Wave of Conviction will always apply fire exposure, increasing all damage dealt for all skills (as we use Avatar of Fire). You can also remove Fortify from this setup and place on the mobility setup (commented below).

4link: Mobility

Whirling Blades - Leap Slam - Faster Attacks - Culling Strike

I'd say Leap Slam and Faster Attacks are mandatory here. The other two gems are up to you. My current setup is Leap Slam + Whirling Blades + Faster Attacks + Culling Strike. Leap Slam is highly recommender for mapping because of cliffs and other obstacles. For the open slots, I've tested gems such as Whirling Blades, Culling Strike, Fortify, Endurance Charge on Melee Stun, Summon Ice Golem, Vaal Grace, ... It highly depends on your playstyle.

4link: Auras

Herald of Ash - Flesh and Stone - Precision - Summon Skitterbots

Flesh and Stone is mandatory to guarantee us 100% hit chance. Herald of Ash is extremely strong, clearing packs with its area effect and giving us a "more spell damage" effect. Precision 1 gives us a nice critical strike chance and some awesome Watcher's Eye mods for a 22 mana reservation cost. Skitterbots will slow and shock nearby enemies, effectively increasing our damage output.

4link: Utility

Vortex - Arcane Surge - Vaal Righteous Fire - Increased Duration

Vortex will deal fire damage because of Avatar of Fire, but its AOE will still chill enemies, making it safer for us and increasing our critical strike chance (from Cospri affix). As we'll be casting Vortex a lot without mana problems, Arcane Surge will have a 100% uptime (be sure not to overlevel Arcane Surge, keep it at a level you can guarantee its buff with a single Vortex cast). Vaal Righteous Fire is here to benefit from Increased Duration.

Another possibility is to run a CWDT setup instead of an Arcane Surge setup. For example, you can run Cast when Damage Taken 1 - Immortal Call 3 - Increased Duration - Vaal Righteous Fire.
With this change, we lose some "more spell damage", mana regen (which we don't really need) and chill effect (from Vortex) and gain some physical damage reduction, which we lack. The damage loss is quite considerable (my numbers changed, but it's around 11% DPS loss - we still keep around 7.4m DPS) but we gain some toughness. I still like Arcane Surge but that's an option if you like it safer (and maybe if Vortex lags you). :P

Gear Setup

Weapon

Cospri's Malice: Not much to say about them. Just need to say they're not mandatory to start running the build. Check the leveling section for more info.

Helmet

I'd recommend you to get either a Devoto's Devotion or a Starkonja's Head. They both gives us many useful status and the dexterity we need to use Cospri, and are relatively easier to find an enchanted one (I said easier, not cheaper :P). A rare helm is also an option. For rares, there are many useful affixes. Try to look for dexterity, life (maybe % maximum life too, but it'll be harder to find), resistances, and possibly a "nearby enemies have -9% fire resistance". Use the craft to fit your resistances or dexterity, or put a “% of physical damage from hits taken as fire damage”.

Chest

I'd recommend a rare evasion chest (preferably Assassin's Garb base for more evasion and implicit, but any evasion one will work). Look for an elder chest with "attacks have +#% critical strike chance", it's a great affix. Other useful affixes are life, % maximum life, resistances, dexterity and "recover #% of life on kill". You can craft life, % maximum life/mana, resistances, dexterity, chance to dodge spell hits and chance to block attack damage. Also, be careful if you're buying a strength chest, as we are low on this attribute!

If you're interested, uniques that are possible options are Carcass Jack and Loreweave.

Gloves

Look for rare gloves with life and resistances. If you have an open prefix, you can craft "% increased damage during any flask effect". If you have an open suffix, you can craft "#% increased global critical strike chance/#% increased elemental damage if you've dealt a critical strike recently".

Boots

Look for rare boots with life, high movement speed (I'd recommend 30%+ but maybe 25% is ok) and resistances. If you have an open prefix, I'd recommend a "cannot be frozen" craft. It's pretty expensive but you'll no more rely on flasks for that, freeing a suffix from one of your flasks. Cooldown recovery speed is also an option to look for, but it'll make the item much more expensive, só I recommend you to leave it for the belt.

Belt

Look for a rare shaped Leather Belt with life (affix, not implicit), resistances and at least 14% cooldown recovery speed. You can craft % increased damage, resistances or strength, depending on your needs.

Amulet

The Effigon: Mandatory item to run the build. We also benefit from its dexterity, fire resistance, evasion and fire penetration. Try to get one with a "0.5% of fire damage leeched as life" corruption for sustain. Until then, replace the Elemental Focus on Ice Nova setup for a Life Leech Support (placing it on Frostbolt is bad because Frostbolt doesn't always hit a monster before you kill it).

Ring

For your rare ring, I'd recommend a resistance base (such as two-stone, topaz, ...). Get one of them with "curse enemies with level # poacher's mark on hit". Other affixes to look for are life and resists. Also look for one open prefix. You'll then craft "channeling skills have -3 to mana cost", reducing Cyclone mana cost to 1. It's quite a filter, but I must say this single equip might guarantee that you can run even no regen maps withour any mana problems at all, while also gives you a lot of life regen (Poacher's Mark life on hit + Cyclone attack speed) and frenzy charges.

Flask

Divine Life Flask or Eternal Life Flask

• Prefix: Bubbling/Seething/Panicked (up to your preference)
• Suffix: of Staunching

Quartz Flask

• Prefix: Chemist's
• Suffix: of Heat (if you don't have boots craft), of Reflexes (if you do)

Experimenter's Quicksilver Flask

• Prefix: Experimenter's (Alchemist's prefix is also good but lasts for almost half the time)
• Suffix: Adrenaline

Diamond Flask

• Prefix: Experimenter's
• Suffix: of Warding

Eternal Mana Flask (replace Quartz Flask for "no mana regen" maps)

• Prefix: Enduring
• Suffix: same as Quartz Flask

The Wise Oak

To be sure you're using it right, check your resistances on your character sheet. The higher value (between parenthesis) is your uncapped resistance. To benefit from it, you need your highest uncapped resistance to be fire. To fully benefit from it, you need all of your uncapped resistances to be equal. In this case, all elements will both be highest and lowest, meaning that you will receive less damage from all of them (and still deal more damage with fire).

For more gear details, see poe link

Enchants

Helmet

15% increased Cyclone attack speed (possibly the best option, makes it even easier to get to 15 APS but it's quite expensive)

Wave of Conviction's Exposure applies -6% Elemental Resistance (it seems quite rare, but increases your damage for every skill, so it's an option if you're done on APS)

40% increased Ice Nova damage (not so cheap but it's an alternative if you're low on currency)

Boots

Damage penetrates 10% resistances if you haven't killed recently (it'll basically only apply to bosses, but the rest will die too quickly, so it's by far the best option)
Adds 45 to 68 Fire Damage if you've Killed Recently (will basically only increase our damage during clear, but is still a strong option)
Those two are the strongest ones but there are some other options, not too strong but useful. Life regen, spell dodge, attack dodge, attack and cast speed, leech, movement speed. They're all good enchants you can opt to keep while equipping your character.

Gloves

I recommend "of Reflection" (creates a copy of you) or "of Spite" (nova of projectiles leaving chilled ground) enchantments. They both help your survivability. Still, I'd say this is more personal than the others, so feel free to pick whatever you want.

Leveling

I've put some leveling trees on POB (link is on top of post) to guide you. Remember that you can always personalize it!

Leveling with this character isn't awesome, but it's quite good. You'll play as a default caster until Merciless lab, when you can easily change to COC. I recommend changing only after Lab, as you'll pick some cool stuff on ascendancy.

Choose between leveling with Freezing Pulse or Frostbolt as your main skill (I prefer Pulse because of projectile speed and crit chance). Also be sure to get Frostbolt, TWO Ice Nova and Wave of Conviction gems (your damage gems) ASAP and keep leveling them to use once you turn to COC.

During your leveling, don't overlook gems, just pick the supports that most increase your main skill tooltip DPS. Use a setup with Frost Bomb - Spell Cascade - Arcane Surge for increased damage and mana regen. Keep Arcane Surge in a level such that a single Frost Bomb cast would already give you the buff. Also be sure to get Level 1 Precision on Act 1 (can run it from the moment you get it), Herald of Ice on Act 2 (link it with Onslaught, maybe Ice Bite, if you want to) and a First Snow/Frozen Trail unique jewel (depending on your skill) on Act 5. If you got the currency, buy a second one for an even easier clear. Herald of Ash also increases your spell power, it's a good gem for leveling and you'll also use it on the build. Other gems choices (such as curses, other auras, ...) are up to you. Just be sure to be comfortable with your mana.

For the first two labs, as stated earlier, get Opportunistic first (will speed up your leveling), then Ambush and Assassinate (Unstable Infusion isn't the best option for Cruel lab).

After Merciless Lab, you can change to COC, so there's a quite big change. Get Unstable Infusion and replace Ambush and Assassinate for Deadly Infusion (if you lack refunds, go back to acts and do the refund quests). Get a COC setup with Cyclone, Cast on Critical Strike, Frostbolt, Ice Nova (add Increase Critical Strikes and Faster Attacks for 6link - Additional Accuracy is also an option in the beginning). Get a Frozen Trail jewel so your Frostbolt has additional projectiles. Also get two weapons with good APS and critical strike chance, such as Widowmaker (pretty cheap) or Brightbeak (still kinda cheap), so you can start spinning around with many casts per second (there might be other good uniques I don't remember right now, and you can also look for one-handed rares). Don't pick Avatar of Fire until you get two Cospris (also don't get fire damage nodes yet).

At this point, you can start mapping, or farm a bit on Blood Aqueduct (which can also you get you a Tabula). The 6link is quite important, even if doesn't have useful affixes (like a Tabula). Get one, then your next focus is to get one Effigon (with Flesh and Stone) so you can forget about accuracy for the rest of the league. Depending on the price and on your clear speed, either buy an Effigon, or buy an Ignomon and a Blind Faith prophecy and upgrade it yourself on a Haunted Mansion map.

Once you get your first Cospri, you'll probably lack dexterity to use it. Aside from dexterity on items, you can respec some nodes to get Alacrity and minor dexterity nodes to the right of the tree (near Acrobatics, on the way to Herbalism) to help you fulfil your requirement. Place your second Ice Nova on it (now you have one on Cospri and one on 6link) and save currency for your second Cospri, where you'll put your Frostbolt (remove it from 6link). Place your Frostbolt Cospri on your mainhand and your Ice Nova Cospri on your offhand. Once you're dual wielding Cospris, you're good to get Avatar of Fire. Once you're dealing fire damage only, be sure to place our final gem setups for Cospris and 6link (check gems section or POB build). Run a Life Leech Support on your Ice Nova Cospri setup until you get another source of leech (fire leech corruption on Effigon or helmet).

Don't forget: for last lab, get Mistwalker!

Map Mods

There are only a few mods for us to comment. First, reflect elemental is quite impossible for us. Although we don't take reflect damage on critical, our spells' critical strike chance isn't high enough to guarantee us a safe run. Thus, I always avoid reflect elemental maps. The good news about it is, this is the only mod we can't really do!

The first one I'd warn you to be careful about is no mana regen. I recommend you to always bring an Enduring Eternal Mana Flask on those maps (I replace Quartz Flask because I have immunity to freeze on boots; if you depend on it to avoid freezes, use an Enduring Eternal Mana Flask of Heat). If you have a Poacher's Mark ring, you can possibly do this mod even without a mana flask (be careful on using movement skills). Just be careful not to face a map with both no mana regen and hexproof without a mana flask, or you'll certainly die.

The second one is cannot leech. It can be boring and possibly deadly if your only sustains are leech and your life flask (if you have the "recover life on kill" mod, you'll probably be ok).

The last one is less maximum resistance mod. On red maps, it goes up to 12% less maximum resistances. This means that instead of 75% resists, we have 63%. Instead of taking 25% damage, we take 37%. It's almost a 50% increased damage taken. So be careful about elemental damage when running this mod!

The other map mods aren't really any harm for you. Curse mods are avoided by our Diamond Flask of Warding. Monsters with increased critical strike chance/multiplier isn't a deal because we don't take extra damage from crits. Nemesis/Bloodline can be a problem if you doesn't react fast enough to remove Corrupted Blood (if you have immunity due to jewel corruption, just ignore it). Reflect physical just doesn't happen because we use Avatar of Fire. Elemental Equilibrium is a little bad and we can't avoid it but we still steamroll everything anyways. Most of the others don't even deserve to be mentioned here.