Haunt
Haunt appears on creatures, instants, and sorceries. When a creature or spell with haunt would be put into a graveyard, instead it is exiled “haunting” a creature. Haunt allows a player to use an effect twice: once when the spell is played (or the creature enters the battlefield), and once when the creature it haunts is put into a graveyard. Haunt appears in Guildpact and is the ability associated with the Orzhov Syndicate.
Here I envision a tactics game. Each unit can have an ability that either helps an ally or hinders an enemy after they die. The simplest example would be a unit that once killed haunts their killer and deals damage to them every turn. I imagine there would also be a fun opportunity for a class whose abilities revolve around interacting with these haunting units.