This is an idea I had for a Warlock ability - probably a high proc chance with moderately long internal cooldown granted by an item.
Expert Warlocks are such masters at applying curses that trivial matters like death offer no respite for their victims.
The Warlock applies mild health degeneration for 20 seconds. If the target dies while affected by this curse, an immobile spirit will appear at the victim’s location for the remainder of the duration. Maddened by its recent demise, it will attack its former allies at range.
While this risks encroaching upon the Necromancer’s design space, it’s differentiated by the inability to command the spirit, pet/minion bonuses do not affect it, and it cannot move. Being the result of a curse also ties it thematically to the Warlock.