The soul of Moreina grows more discontent with every life claimed by her corrupted host, Blood Raven. She gains stacks of Torment, up to 100, for each of Blood Raven's kills (1 for minions, 10 for heroes), and at 100 stacks of Torment she will be able to burst in a torrent of energy, the effect of which depends on Blood Raven's active Elemental Essence.
Lightning Storm- A lightning storm will surround Blood Raven for 6 seconds. A lightning strikes will deal 90 (+18/lvl) to a random enemy hero within a large radius upon activation, and every 2 seconds after.
Ice Trap- Ice engulfs Blood Raven, leaving her immobile, but immune to all damage for 4 seconds. Shards of ice whirl around her, dealing 20 (+5/lvl) damage per second in a moderate radius and slowing all enemies affected by 30% for 2 seconds.
Poison Cloud- Burst in a cloud of poisonous gas that covers a moderate radius and persists for 4 seconds, dealing 60 (+15/lvl) damage over 4 seconds. Enemies poisoned by the cloud will take 25% more damage for 2 seconds.
I would add that Frosty Armor (Q) seems to me kinda OP and I think it could be connected to all essences.
Could be named Elemental armor. With frost essence it slows attacks, with poison essence it gives back to attacked weak dot, with lighting essence you get reduced duration of CC by 50%.
And I then you could change effect of lighting essence on Q to something more offensive like: Dealing 4 hits on same target in 10 seconds makes him vulnurable for 4 seconds.
Also I don't like generic talents and even Blizz is getting rid of them lately, thus it could be nice idea to make them more unique.
I've got a big issue with Tortured Soul though, as of now, it's way too hard to get the stacks. Imo you should give it a CD and a mana cost to launch it. Kills nearby would lower its CD by Xs for minions and mercs and Ys for heroes. It still fits your basic idea, but it way more usable.
Also :
3. Potent Poison (Q)- Basic attacks return 10 mana per hit while Essence of Poison is active.
---> Way too good, more mana on kills or health on hit could be better here (but you already did that in tier2)
1. Burning Soul (R1)- Each stack of Torment increases all damage by .1%.
--> Seems not worth it at all
She'd be really fun AND hard to play. Hope this helps. :)