The main mechanics of this build are as follows:

We take the Crucible node for Purifying Flame that converts 100% of it's fire damage to chaos, and causes it to create Profane ground instead of Consecrated ground. We then convert from Phys to Fire (Purifying flame inherently has a 50% conversion). This allows for some interesting scaling:

1. We can scale Herald of Ash buff effect everywhere we can. There's a crucible node that gives 45% increased buff effect at the cost of 25% increased reservation efficiency. We can get this node on each of our weapon and shield and we can counter the increased reservation from the tree, a ring or even a helm enchant.

2. Since our final conversion chain is phys->fire->chaos, scaling chaos damage affects both our main skill as well as the explosions. Stacking unspeakable gifts on clusters means even better pops (I have 3 in the showcase). We can get some free gem levels from supports like Awakened Added Fire and Awakened Elemental Focus, whilst also grabbing the Heart of Destruction forbidden jewels for some juicy AoE and damage.

Here's a clip of the last few waves of a Simulacrum (+47% chaos res on wave 30 btw):

https://www.youtube.com/watch?v=rXFu4bpGaRc

My gear isn't minmaxed at all, but here's my current PoB as used in the showcase: https://pobb.in/xAkXOa_VMa8H