Eye of the Void is a Head Rune added in Phase 3 of WoW Classic: Season of Discovery. This Rune is only available to Priests, and a minimum level of 50 is recommended when seeking out this rune.
Use it to combine the Soul of the Void with the 10 Conjuror's Pendants. Kill the Void Seeker that spawns and the Invocation Rune is yours.