Spectre

Spectre is a good example of an aura whose strength is not its rarity. Legendary tier, 1 in 140,000: roughly 140,000 rolls at 1x luck and 14,000 at 10x, which any long auto-roll session covers. It came with Eon 1-2, needs no biome, and the wiki lists it as obtainable. The in-game text calls it an aggressive hunter that collects weak souls, and the kit follows it.

The player carries a dark scythe whose centre piece looks like an eye, a whirlpool of dark and light blue smoke turns at the feet, three small ghosts circle the torso, the left eye shines light blue, and chains with ancient symbols rotate around the wrists. When it walks, the character moves like a zombie, swinging the left arm and the scythe. The single ability, Soul Cutting Slash on F, raises the scythe, builds momentum and spins a left-to-right slash for 25 damage with a 10 second cooldown. VoicedCynical created it, and Spectre : Requiem is the mutation built on top of it.