Shackled Thrull

Shackled Thrull costs {1} less to cast for each Thrull
you control.
Non-Thrull creatures you control have "When this
creature dies, create a token that's a copy of it,
except it's a 0/2 white and black Thrull."
Token creatures you control can't be exiled.
Eternal servitude, granted by souls tethered to the
mortal realm like abandoned mutts.