Each time Rend deals damage there is a chance a Blood Feather is spawned near the enemy on the ground that you can pick up to gain 1 stack of Owed in Blood for 30 seconds, up to maximum of 150 stacks.
Enemies that die with your Rend active on them have a 25% chance to create a Blood Feather.
When you use your Owed in Blood ability, all stacks of Owed in Blood you have accumulated from Blood Feathers are applied to the targeted enemy as Rend stacks.
Gunde