[3.19] CoC Ball Lightning Scion

The main skill in this build is cyclone - cast on critical strike (CoC) - Ball lightning. You channel cyclone, and every time you crit, ball lightning is triggered.

In addition, we use Vortex as a support skill. Importantly, vortex is an instant spell, which means that you can cast it while channelling cyclone. Thus, the gameplay is smooth even though we use two active skills.

Doryani's Prototype

Nearby enemies' lightning res becomes -200%, which is very powerful.

Damage conversion

The downside of Doryani's Prototype is that you are weak against lightning damage. This build uses new rings to convert 50% of lightning damage from hits to fire and cold. The other 50% of lightning damage is taken as chaos by using the Divine Flesh keystone offered by Glorious Vanity. Thus, you take no lightning damage from hits, and only 50% from lightning DoTs.

Projectile speed

With the reflection mechanics introduced in 3.19, it is now possible to significantly reduce projectile speed of ball lightning. With the amulet and 20% reduced projectile speed from the passive tree, ball lightning can hit an enemy 13 times per cast/trigger.
I recommend you get at least 86% reduced projectile speed in total, although PoB says you need way less than that. The reason is that PoB assumes you are self-casting ball lightning, which isn't the case in this build.

Brittle ground

Enemies on brittle ground are brittled, increasing your base crit chance for both attacks and spells by +2%. We can actually get more than +3% base crit chance by taking some "increased effect of non-damage ailments" stats. Because your ball lightning's projectile speed is so slow, each ball lightning projectile hits an enemy 13 times even if you are standing (spinning) on top of the enemy (previously, you could not stand on top of the enemy to deal max damage). Thus, it is very easy to inflict brittle to your enemies.

Power Charge Generation
We need power charges to consistently trigger ball lightning. This build uses the Stormrider notable, which is offered by medium or large cluster jewels.
Stormrider grants you 10% chance to gain a power charge when you shock a chilled enemy. How do we inflict shock and chill on enemies? Because Ball lightning is a lightning spell, we can naturally shock enemies. To inflict chill, we use vortex. Vortex creates a chilling area, and enemies on it are chilled, regardless of their life. The chilling area can chill enemies even if you are wearing Doryani's Prototype, which makes you unable to deal cold damage.

Because our damage source is ball lightning, you want to trigger it as often as possible. ball lightning is triggered when you crit with cyclone. This means that the attack rate and crit chance of cyclone are important factors that determines how often ball lightning is triggered. A high attack rate and crit chance allows you to trigger shock nova more often.

However, this does not mean that you want to make the cyclone attack rate as high as possible. The reason for this is that cast on critical strike (CoC) has a cooldown of 150 ms, which means that you want to attack less often than this cooldown. You might think the best attack rate is 1/0.15 = 6.66 per second, but this is incorrect. Every action in this game happens at a constant interval of 33 ms (1 server tick), and as a result, the cooldown of CoC behaves as if it is 165 (33*5) ms. With increased cooldown recovery rare (CDR) stats, you can reduce this cooldown. The summary is given below:

With 0-13% CDR, your optimal attack rate is 6.06 aps.
With 14-51% CDR, your optimal attack rate is 7.56.
WIth 52%+ CDR, your optimal attack rate is 10.1.

Note that there is no difference between, for example, 14% CDR and 20% CDR. Also keep in mind that the "9-12% cooldown recovery rate" crafting mod on belt has no effect if you don't have other sources of CDR since 12% CDR is the same as 0% CDR.

14% CDR is recommended when you start the build. 14% CDR can be obtained from belt alone, but due to the low price of awakened CoC gems in this league, the best option may be to buy the awakened gem. Level 3+ awakened CoC gives 14%+ CDR. Crit mastery in the passive tree gives +3 level to all crit gems (including CoC), so even a level 1 awakened CoC gem gives you more than 14% CDR.

For 52% CDR:
1) awakened CoC gem with the crit mastery: 31% (level 5), 34% (level 6)
2) boots implicit (up to 10%)
3) belt (up to 12% with crafted mod, up to 16% with unveiled mod, up to 20% with shaper/crusader mod)
4) abyss jewel

Read this reddit post if you want to understand the basics of CoC.

mana cost
Mana cost of CoC builds can be solved in two ways: (1) reduce the mana cost, and (2) use "gain x mana on hit". This build uses (1) because it not only solves the mana problem but also allow you to use wrath - divine blessing for (almost) free, which is a big damage boost.
Mana cost reduction can be obtained from:
-passive tree (15%)
-divergent inspiration (46% with the +3 level crit mastery)
-flask suffix (32% with 30% increased flask effect)
-jewel (up to 5% per jewel)
-ring/amulet (5%)

97-99% mana cost reduction is your goal. Don't get 100% or more because then you can't gain inspiration charges. Alternatively, you can use "-x to mana cost of non-channelling skills" craft on ring/amulet. This is an easier solution as you don't need as much mana cost reduction. However, you can't use the wrath/divine blessing setup, meaning you lose a lot of bossing dps.

ailment and stun immunity
You are immune to chill and stun because of the ascendancy node. Freeze immunity can be obtained from boots or pantheon. Shock immunity (kind of) can be obtained from pantheon (60% reduced effect) and gloves or rings (40% reduced effect).

Sceptre

Shield


Helmet

Body armour

Gloves

Boots

Belt


Amulet


Ring


Jewel


Flask

See the gear section and PoB.

Bandit: Kill All.
Pantheon (Major): Solaris.
Pantheon (Minor): Garukhan for shock effect reduction. You can also choose the one that gives immunity to burning ground.