This build is based on Rain of Arrows. Rain of Arrows is a ranged attack skill that fires a number of arrows in the air, with each of them landing to deal area of effect over a short period of time.

This build is created to be pretty well-rounded. It has good clear, great survivability, and really strong single target.

Quick Jump

• Pros & Cons

• Skill Tree 

• Gems Setup

• Gear Setup

• Leveling

Pros & Cons

Pros

• + Good clear
• + Good single target (Elder, Shaper and Uber Elder down)
• + Great survivability, with over 10K ES, three curses, and very quick start of ES recharge
• + Cheap to get started - all required uniques are cheap, and ES gear is on average cheaper than their life counterpart
• + Can do all map mods
• + No need to worry about resummoning your minion, he's invulnerable to damage and summons/unsummons itself
• + Fun to play ("It's like a minion build but not boring" ~ Rockell)
• + Can take everything down up to Shaper without heavy investment (I'd say on a 1 or 2 ex budget), and can take down Uber Elder with some further investment (a faster start of ES recharge Watcher's Eye is almost mandatory)

Cons

• - The Agony Crawler doesn't have a perfect AI, and will sometimes get stuck behind walls, altough this can be solved by having a Convocation gem (now that it's instant cast, it's confortable to use)
• - Requires one six-link for HoA, and one for sustaining poison - you can play with a five-link for your poison setup, but you'll only have two curses
• - Off-colours can be hard to get on chest (4 green 2 blue)

Skill Tree 

Ascendancy

Bandits

Kill them all.

Easy choice: the only bandit worth helping on some builds is Alira, and since we do not profit from the crit multi, the two skill points are what we go for.

Pantheons

The Pantheon List

• Major gods
• Minor gods

Passive Tree

Gems Setup

Herald of Agony + Damage on Full Life Support + Minion Damage Support + Vile Toxins Support + Vicious Projectiles Support + Added Fire Damage Support

Damage on full life does apply to the Crawler's attacks (confirmed by PoB). Since the Crawler is immune to damage, it will always be full life. Concerning Vile Toxins, the Crawler doesn't deal any poison damage, but we poison enemies, which makes the Crawler deal up to 45% more damage against enemies affected by 9 poisons. It provides a nice damage boost against target we poison a lot, i.e. in single target.

Note on level 21 Herald of Agony

Virulence grants several buffs per stack to the Crawler, one of them being attack speed. It goes from 2% at lvl 1 up to 3% at lvl 20, and at lvl 21 it goes up to 4%. That means that the overall attack speed buff from virulence increases by 33%, which is huge. In PoB, a lvl 20 gem displays around 750K dps, and with the lvl 21, it's over 1M. Plus, quality is useful on HoA, but not mandatory (it's minion movement speed), and there are cheap-ish lvl 21 gems (I bought mine for 1ex mid league), so it's a valuable late game upgrade.

Rain of Arrows + Poison Support + Curse On Hit Support + Temporal Chains + Enfeeble + Projectile Weakness

The Poison Support gem, along with the HoA buff, puts us at 80% chance to poison, which is more than enough. If you can only have a five or a four link for this, drop one or two curses, according to your preferences, although I wouldn't recommend taking out Projectile Weakness when packs have a decent density.

A six-link Herald of Agony reserves a lot of mana, so we only have the mana for another aura. Of course, we use Discipline

Note that, in order to use both Herald of Agony and Discipline comfortably, you'll need to either have a lvl 3 Enlighten on Discipline, or take the 4% reduced mana reserved node near Chaos Inoculation.

Summon Chaos Golem + Summon Lightning Golem

For the Golem, you can use either a Chaos or a Lightning golem. I started out with a Lightning golem, but I found I was always at maximum virulence anyway, so I opted for a Chaos golem to try and compensate the fact that we have no armour.

Blink Arrow + Faster Attacks Support

For mobility, Blink Arrow linked with faster attacks is perfect.

In the Cast when damage taken (level 1) setup, Immortal Call is used to allow the ES recharge to start more easily, and Phase Run for invisibility / mobility.

Immortal Call + Cast when damage taken + Phase Run + Increased Duration Support

For the other gems, feel free to use whatever you like using (Fortify setup, golem in another, high-level, CwDT setup, ...)

The following is an optional setup for single target, if you struggle to sustain Virulence with Rain of Arrows (see next part for more details).

Barrage + Poison Support + Lesser Poison Support + Faster Attacks Support

Gear

There are a few key uniques to this build, but they are all really cheap (unless you choose to go LL instead of CI, in which case you need Shavronne's Wrappings).

Weapon

Quill Rain is an important part of the build, providing us with the necessary attack speed to stack Virulence without needing to invest in attack speed at all. The high attack speed does not bring any mana issues, since Quill Rain grants a bit of mana gain on hit, and Rain of Arrows is essentially in a three-link (RoA, Poison, Curse on hit).

Soul Strike is the first and main reason we use a bow to apply poisons. 80% faster start of ES recharge, what more is there to say? It provides incredible survivability, both while mapping and while bossing.

Helmets. 

There are two helmet enchants for the Crawler: reduced mana reservation and two additional projectiles. The mana reservation reduction isn't enough to use another aura, and I really feel like the Crawler doesn't need any additional projectile, so I wouldn't invest in it.

This makes the search for a helmet very flexible. You can ignore helmet enchants altogether and just look for a helmet with good ES and the resistances you need. Otherwise, there are several utility enchants that come in handy

• Increased Immortal Call Duration (34% Merciless Lab, 36% Eternal Lab)
• Increased Enfeeble/Temporal Chains Curse Effect (20% Merciless Lab, 30% Eternal Lab)
• Rain of Arrows has (10% Merciless Lab, 15% Eternal Lab) chance to fire an additional sequence of arrows
• Increased Effect of the Buff granted by your Chaos Golems (75% Merciless Lab, 100% Eternal Lab)

Armours

As an alternative to CI, you can go LL and play with Shavronne's Wrappings.

The point is to be able to use your life to reserve mana for the aura Hatred, which grants a lot of damage to the Crawler. However, you will have lower ES (the maximum ES a Shav's can have is 351; for comparison, you can get 500 ES chests for a few Chaos), and getting a 5L/6L Shav's is more expensive than getting a 5L/6L rare chest.

In a nutshell, CI is tankier and cheaper than LL, but LL has more damage.

Belts

Bated Breath is basically the best in slot for all ES recharge builds. It gives a lot of ES, as well as a lot of ES recharge rate - it overcompensates the ES recharge rate reduction of Soul Strike.

Rings

Dream Fragments is not necessary, but the immunity to Chill and Freeze makes the Uber Elder fight so much more comfortable. I highly recommend it.

In all other slots, you can use unique items you like or rare items, but there should be nothing too expensive.

Leveling

Before starting

There are some gems you'll need that aren't given as quest reward and can't be bought as Witch. Therefore, you will need to get the following gems before starting. For each gem, I noted where you can get it in the story and as which class, but remember that if you have a character that has access to act 6, you can buy all gems from Lilly at act 6 after clearing the Twilight Strand, as well as in act 10, or after completing Siosa's quest in act 3.

Gems to grab before starting 

• Lesser Poison support [lvl 1] (on the Twilight Strand as Shadow)
• Pierce support [lvl 1] (on the Twilight Strand as Ranger)
• Rain of Arrows [lvl 12] (after killing Merveil as Shadow/Ranger/Duelist)
• Herald of Agony [lvl 16] (after killing Fidelitas and saving Helena as Shadow/Ranger/Duelist)
• Poison support [lvl 31] (only obtainable from Lilly or Siosa)

It is not a gem, but I recommand you buy a Quill Rain before starting. More links are available in the Gear section, but if you're too lazy to scroll.

Gems & key gear

You can start leveling by using minions, starting with zombies at lvl 2. Then, when you get raging spirits (after the Mud Flats), you can actively summon those while your zombies take the damage for you, as well as skeletons if you prefer (after killing Brutus).

At level 16, you can equip Herald of Agony! You can equip your Quill Rain to use Rain of Arrows + Lesser Poison as well as your Herald of Agony + Pierce (+ Minion Damage/Minion Speed/etc.). The agony crawler will carry you all the way to the end game. At level 24, remember to grab Discipline.

As for curses, the order of priority is Projectile Weakness > Enfeeble > Temporal Chains. Projectile Weakness removes the need for a Pierce support gem, freeing a socket. To apply your curses, you can

• Curse on hit with Rain of Arrows if you have the sockets (the priority is Rain of Arrows > Lesser poison/poison > Curse on hit > Curse gems).
• Self cast them if you don't.

Skill tree

Start by grabbing Lord of the Dead (minion damage and life) and Arcane Focus (ES & ES recharge rate) just next to the start of the Witch tree. Then, go the right side, then down to the start of the Scion, and grab Foresight. You can then grab Gravepact, to the left, and go up to Spiritual Command.

(If you want more early minion damage and less ES, you can skip the part about going near the Scion start and go straight left from Occultist's Dominion and refund the points later)

Then, go up near Avatar of Fire (don't take it! Just go near it), then down toward Resolute Technique. Now you won't ever miss your shots! That means more poison, so more virulence. If you feel like you miss too often while leveling before Resolute Technique, you can use an Additional Accuracy support gem on Rain of Arrows.

After that, go up and grab Whispers of Doom, allowing you to apply two curses (you should hit that point a bit after you get the curse gems). You can then curse enemies with two curses, which should be enough. (If one curse is enough for you, you can grab it later). Then go left toward Redemption and Spiritual Aid. If your zombies die too often, you can grab Sacrifice or Righteous Army and refund it later. The rest is pretty much completing the skill tree according to your own preference.

Life or Energy Shield

ES leveling

Always have some recent ES gear, especially body armour (unless you have a Tabula Rasa). ES armour can be bought from vendors, and an Essence of Woe will guarantee some ES on it and is very cheap (less than 1 alchemy for essences below lvl 60). On rings/quivers, I advice to look for as much elemental resistance as possible.

In the skill tree, go toward Essence Surge as soon as possible. It makes the ES recharge faster and happen sooner!

After your first lab, your ES recharge can't be interrupted for 4 seconds after starting, which will make your life a whole lot easier.

Useful uniques

• Tabula Rasa (lvl 1): easy 6L. Around 15c. Here's a link to buy yours
• Crown of Thorns (lvl 1): lots of ES as early as possible.
• Bated Breath (lvl 22): lots of ES & ES recharge rate.
• Shavronne's Pace (lvl 32): some ES and 35% move speed when you're on full life.
• Martyr's Crown (lvl 52): fated version of Crown of Thorns. Even more ES.

Life leveling

If you don't feel comfortable leveling with ES, you can level with life instead. Grab whatever life nodes you want to grab in the skill tree, and refund them later on.

Useful uniques

• Tabula Rasa (lvl 1) ez 6L. Around 15c. Here's a link to buy yours.
• Goldrim (lvl 1): lots of resistances, 'nuff said. 
• Meginord's Girdle (lvl 8): lots of life. 

POB link: https://pastebin.com/jHPLkTub