The Edge, Carbine, and Crusher are made from the very essence of the Apocrea. As such, killing an enemy with then should have its own...possessive effect.
What I'm proposing is this:
Whenever a Monster (excluding obvious ones like Bosses, Souls, Battlepods, Wheel Launchers, or Phantoms) is killed by an Obsidian weapon, a brief scene without collisions plays that opens up a rift underneath the monster, immobilizing them, and drags then down into it with the Hands of the Harvester's attack. The Apocrean Gaze sound plays shortly before the monster vanishes.
The affected monster is placed into its "idle" state during this.
Trojan and Tortodrone Shells, when broken by an Obsidian weapon, shatter and sink into a rift, but hands do not appear. Note that the remnant must be destroyed rather than the monster itself.
Aside from this, the monster dies normally, leaving behind loot. Just thought it would be fun to inadvertently become the "Harvester's henchman" through trying to defeat it.
(Small addition: The Grasping Aura causes this animation to play regardless of weapon used so long as it is visible on the player)