This serves as a further illustration of the concept from the last video: https://youtu.be/E5u3u1IXe00
How it works:
Unearth is linked with Archmage support. By reserving some proper portion of mana, we control the mana spent by unearth to slightly above 100 per cast. Then we use Kitava's Thirst to Trigger a 4L DD of chain reaction.
To mitigate the mana cost, we use "% chance to recover 10% mana when you use a skill" mod on mana mastery, watcher's jewel mod while affected by clarity, charms, and the new charm-related jewel from 3.23, to stack the chance of recovery to 45%~50%.
At max roll:
Charm: 10% x3 = 30%
That Which was Taken jewel: 10%
Watcher's Eye (clarity): 15%
Mana Mastery: 10%
Amulet with Essence mod: 10% (12% with catalyst)
Max Possible: 77% chance to recover 10% of total mana when you use a skill.
Archmage Support: 5% of unreserved mana as base mana cost, making it possible to spend 100+ mana per cast if we have some proper portion of unreserved mana. If we want to reserve more mana, we can also have increased mana cost modifiers from the passive tree (between Scion and Templar), the quality of the archmage support gem, and the Fevered Mind jewel, so that we have a smaller unreserved mana pool but the mana cost is still above 100.
Current setup:
Dialla's body armour: Unearth-GMP-GV-Empower-Enhance-Archmage
Kitava's Thirst: DDCR-Hypothermia-Awakened Fire pen-Combustion
I think this mechanism can also be abused to utilize corpse pact to get the 200% inc cast speed for any spell. For example, have a 6L in body armour that costs 100 mana, and use Unearth-GMP-GV-DDCR (or Desecrate+DDCR) on Kitava's Thirst to consume corpses quickly so that the body 6L can benefit from the extra cast speed.
Make sure to get at least a 2% cooldown recovery rate from belt/boots/abyss jewel so that the actual trigger rate can be three ticks instead of four (i.e., 0.099 seconds instead of 0.132)!
Current POB (not optimized!): https://pobb.in/FEfQvFr5rl7_