A voracious Tomb Swarm Leaves Nothing to Waste.
From RoR Wiki
Order and destruction
Find a "Pile of Bones" (7000;23400) in Land of the Dead, there are some piles at different locations. When you found one, interact with, then a tomb swarm will spawn. Kill it to have the unlock.
Reward
Trophy : Tomb Swarm
XP : 1974