Recently I made a thread regarding Relic of the Pact and I believe I got to some conclusive PoB.
Initially, I went in the Reddit post direction of going for Hierophant - the fellow unfortunately gave up on the build quite quickly(lvl 83sh~) so I had to pick up from there. Maximizing AoE, being able to reserve 99.9% life. The problem with that is, it is difficult to stack Life(for damage), ES(for alternate health pool) and mana (for AoE and MoM). In the end you end up short of damage or ES pool.

Since then, I switched to Chieftain and noticed how powerful the skill scales with phys as x% bonuses and conversion. However, then I realized the AoE would be too puny and the cast rate is also poor. Who can give the skill both on top of other goodies? - Elementalist.

The problem was, how do you exceed 49.89% reservation? If you use lvl 1 Arrogance, Lifetap and a 140% support, you end up with 49.89% reservation and there is but one way to increase reservation of skills from there - Memory Vault. With it and a level 12 Arrogance, the reservation is exactly 50.01%, which allows you to benefit from Low Life, use Petrified Blood(since it is not an aura, you can use it with Supreme Ego) and not overcast the ability for 99.9% of your life. It's not an ideal solution, but you gain a lot of armor from reserving 71% of your mana so it's not too bad. You lose AoE of course, but you cast twice as frequently, gaining huge cast rate scaling and you can run and tap which is super useful for Ultimatum.

The ability unfortunately has very rigid itemization - you want a lot of life and cast rate and added damage being slightly behind and are stuck with 2 essence worms at all times, the uniques are very fire res happy so you end up needing some cold and lightning res from other sources.

Here's the progress so far, I'm rather proud of this 10 mil(11.1 mil top) DPS Relic of the Pact build. https://pastebin.com/TRZDdMdp

I welcome further polish to this build as your feedback so far has been very helpful.

Edit: I've made a gameplay showcase: https://www.youtube.com/watch?v=RfbRd-rxDtc
I adjusted my build since: https://pastebin.com/hHWBxYaj