Thanks to the wealth of possibilities unlocked by Crucible, I knew I wanted to play a build that could only be played this league. This build can only be played in Crucible for two reasons - the rolling magma AOE mod, and the base mana regen afforded by depth 2 mods.

Showcases + POB

pob: https://pobb.in/93LpjHsQdiVB

Uber Maven: https://www.youtube.com/watch?v=KZbe9Nt-K-c

Mapping: https://www.youtube.com/watch?v=IB5pjH_XOVI

Key mechanics

Crucible mod Fireball and Rolling Magma have 100% more Area of Effect / Modifiers to number of Projectiles do not apply to Fireball and Rolling Magma - Rolling Magma bounces are capable of shotgunning, but different projectiles on the same "bounce number" cannot shotgun with each other. However, the same projectile on different "bounce numbers" can shotgun. So projectile scaling only helps with coverage, whereas AOE helps with both coverage and single target damage. We use two copies of this modifier, which I think stack multiplicatively to 300% more AOE.

Hierophant Sanctuary of Thought and Mine Mastery - You gain 5% increased Area of Effect for each Mine combine to add even more increased AOE, up to 205% on my build but in practice more like 150-190%. This is easily enough to let us skip El'Abin's Visage.

Timeless Jewel The Traitor, from Brutal Restraint, allows us to upkeep two magic flasks and a Lavianga's Spirit without going Pathfinder/Scion or anywhere near the Ranger area of tree.

Mana stacking with high Mind over Matter effect, as well as many sources of mana regen, enable Indigon + Arcane Cloak to scale offenses and defenses massively.

Offense + Defense characteristics

Offense is carried by Rolling Magma's shotgun behavior. It's pretty hard to estimate due to Indigon, but I'd say averaging the Arcane Cloak uptime it's close to 40m shaper dps. (POB doesn't understand Minefield or Rolling Magma shotguns, so its calculated dps is much lower than reality).

Defensively, the build relies on a ~13k EHP pool (3k ES, 3k life + 7k mana with 60% MOM effect) with Arcane Cloak, high Sap, Sigil of Power, and the mine mastery to reduce damage. I also have a middling amount of Evasion with blind and enfeeble, as well as over 80% effective spell suppress chance, which together provide excellent nondeterministic mitigation.

Build is immune to freeze, chill, stun, poison, bleed, corrupting blood, and ele weakness capped. Realistically we will not be getting afflicted by any ailments very often due to Extinguish.

Gems

Rolling Magma - High-impact Mine - Minefield - Charged Mines - Slower Projectiles - Elemental Penetration: Of these gems, only Elemental Penetration can really be changed out, and only when you get a high amount of penetration or res reduction from other sources. However, Awakened Increased Area of Effect is the only other support I can think of which makes sense in that slot. High-Impact is the much better mine support as I'm basically holding down the Detonate Mines button at all times while in combat.

Divergent Arcane Cloak - Enhance - Arcane Surge: Divergent is the best alt quality, POB is bugged and thinks it is a substantial damage decrease for some reason. You could link Empower as well, but it barely does anything unless you put it in a +1 gem level item, and IMO that's too much of a sacrifice.

Zealotry: The lone high-reservation aura of choice for the Essence Worm. Provides great damage as well as turning on powerful Watcher's Eye and Rational Doctrine modifiers.

Precision, Clarity: The other auras, also to turn on Watcher's Eye.

Frostblink - Increased Area of Effect - Culling Strike: Thanks to all the AOE investment, Frostblink is massive, which really helps keep it on a low cd. Cull is optional and can be removed to free a gem link.

Sigil of Power - Bear Trap - Increased Duration - Faster Casting: Sigil of Power is excellent and also huge. It's pretty trivial to stay inside it during boss fights.

Anomalous Detonate Mines: Provides a little health sustain. If you find that you're hitting the mine limit, especially if you don't have Volatile Mines, you can instead use default quality. Can also be removed to free a gem link.

Anomalous Assassin's Mark: Can also be Sniper's Mark, and I'm considering switching to that myself.

Cast when Damage Taken - Enfeeble - Cold Snap - Increased Duration: Great mapping defense, Enfeeble helps a ton by reducing enemy damage and accuracy. When combined with blind, it really makes 10k evasion go a very long way to mitigating attacks.

Ascendancy + Tree + Pantheon

Akane's past builds have all been Scions, with perhaps the occasional Pathfinder. This build is a Hierophant instead. The 100% increased AOE from Sanctuary of Thought is too good to give up, and the rest of the ascendancy is fantastic as well (and just got buffed this patch with the Arcane Surge bonuses!) Not using Agnostic gives me a sizable ES pool. In exchange I am forced to give up some flask effect and two magic flasks.

This build can likely be played as Scion or Pathfinder as well, with Awakened Increased AOE instead of Elemental Penetration. You can also path through the Ranger area as a Hierophant to get to the mine clusters, instead of through Witch. I didn't investigate this too hard because it looked worse to me, but it might let you use a third utility flask and cap suppress for better HC viability.

Some interesting things to call out:

• The Chance to Suppress Spell Damage is Lucky mastery is extremely strong if you aren't in a position to cap suppress and you have good nondeterministic mitigation. It's giving me +24% effective spell suppress chance.
• If you don't have very high mine throwing speed from jewels and 2 copies of the mine additional detonation crucible mod, it's probably best to avoid the mine detonation speed nodes (Volatile Mines). At my mine throwing speed, I actually need these nodes to avoid hitting the limit and losing mines.
• The Vampirism recoup cluster is excellent for helping with life sustain, especially if you don't have the arcane cloak enchant. It is not a luxury upgrade, it should be taken very early.
• 50% increased Effect of Non-Damaging Ailments you inflict with Critical Strikes is one of the strongest masteries on the entire tree, my only guess for why it wasn't nerfed is that POB doesn't understand what it does.

Pantheon is Solaris (IMO the best all around major god) and Abberath (IMO the best all around minor god, and its upgrade is practically required to run Exarch altars)

Weapons

The weapons are both crucial and not crucial. For the longest time, I used two haphazardly crafted wands and the build was easily playable. The sceptres I am wearing right now are (for the Alternating Sceptre, at least) the result of many divines worth of Crucible crafting.

If you don't have an Alternating Sceptre, you can consider getting Secrets of Suffering via The Interrogation.

For Crucible trees:

• Fireball and Rolling Magma have 100% more Area of Effect / Modifiers to number of Projectiles do not apply to Fireball and Rolling Magma is mandatory at depth 5 on both weapons.
• Regenerate 1.5% of Mana per second / 20% reduced Reservation Efficiency of Skills at depth 2 helps enormously with mana sustain. You'll probably only be able to afford one of these mods. The other depth 2 mod can be something like spell base crit or no-downside base mana regen (which is what I have).
• 15% reduced Mana Reservation Efficiency of Skills that throw Mines / Mines have a 14% chance to be Detonated an Additional Time is a great damage mod, available at depth 3 or 4.
• Adds 37 to 56 Cold Damage to Spells at depth 1 was an accident, but it turns out to be good enough to apply an almost 2% brittle to Maven. If I had to do this again I'd actively seek it out, or look for a cold damage mod on the explicits.
• If you don't have Secrets of Suffering, you can also try to get Chance to Scorch / Cannot inflict Ignite on one of your weapons for maximum damage.

I crafted the Alternating Sceptre with Deafening Essences of Scorn. I got extremely lucky. The other sceptre was crafted with alt+regal and multimodded.

Gear

Cloak of Defiance + Indigon + Atziri's Foible + Essence Worm + Dream Fragments core is from Akane. I did consider using Nimis but as far as I'm aware, it's actually fairly mediocre on Rolling Magma unless you actively want the randomly directed projectiles. It only bounces back towards the source once, so at 4 chains it's 20% more damage. If you want to play a nimis build, basically any other projectile skill uses it better.

Micro-Distillery is the ideal belt base, but you can use a Stygian as well. It's fractured with chaos res as a luxury, but I honestly don't remember dying to Chaos damage a single time even when I was at -60. Being poison immune (from a corrupted Dream Fragments) is great. I used a similar belt without chaos res for a long time.

The magic flasks were both fairly strict requirements for me. I pretty much needed a stun avoid and an ele res suffix, one way or the other, in order to be stun immune and cap resistances. If you have a repeatable movement skill you can probably use something else instead of Quicksilver. Also the Jade could be an Amethyst (or if you have a lot of res from elsewhere, even a Progenesis), but IMO Jade provides more defensively than Amethyst. Perpetual is probably the only prefix that allows full uptime, and they must be crafted with Reused at the end of this Flask's effect. If you don't have 28 quality, you can temporarily craft flask effect duration on your belt to make up the difference.

Lavianga's Spirit is hypothetically needed as a panic sustain flask, but honestly I rarely press it. I've considered using a third utility flask instead. With Indigon, it will heal you up extremely fast, but only if your mana is not already full.

The Eldritch implicits on the gloves and boots are huge and a big reason why I'm not using uniques in those slots. For the gloves, hinder and extinguish are both great defensive boons that won't show up in POB. For the boots, the bleed avoidance chance, together with the crafted bleed avoidance, is my favorite way to be bleed immune.

The jewels are very much a luxury in my build and you can get off the ground with a lot less, especially in the rare jewels department.

• Unique jewels: The hardest constraint is the Brutal Restraint, which must have Balbala (The Traitor) on it and provide two copies of 20% chance to avoid being Stunned on covered notables. The Watcher's Eye 10% of Damage taken from Mana before Life while affected by Clarity modifier must also be prioritized. It's very cheap by itself. Rational Doctrine is a luxury upgrade and is almost purely damage.
• Cluster jewels: The Large cluster jewel is marginal. The two mediums should ideally have those exact notables.
• Rare jewels: Increased maximum Mana and Mine throwing speed are the two most valuable mods.

Conclusions

Mana stacking builds are temporarily in an extremely strong spot this league, with Crucible patching up the deficiency in mana sustain that has plagued them for years. The mana miner archetype happens to be able to invest very heavily into AOE, which synergizes excellently with the AOE stacking style of Rolling Magma. This build isn't the zoomiest or the most instant boss killer, but it is a very comfortable all-rounder that I am happy taking into just about any content. Despite being pretty frustrated with the implementation of the Crucible league mechanic, I'm in awe at how much it has unlocked build possibilities, and I hope GGG continues to design in this direction.