Generation

Put the "Essence Absorber" block on top of a wither skeleton spawner. You can find one in the "Ancient Nether Temple" structure that spawns in the nether. After around 12 essence points the spawner will turn into a normal skeleton spawner.

Spells

Normal Spell - Death Skull

Shoots a wither skull in the direction you are looking.

Consumes 5 Death Essence.

Sky Spell - Death Helix

Spirals all the mobs around you to the sky and kills them.

Doesn't effect boss mobs.

Consumes 30 Death Essence.

Heavy Spell - Reaper

Shoots a black cloud that kills EVERYTHING.

Consumes 50 Death Essence