At the beginning of your upkeep, sacrifice Breeding Pit unless you pay {B}{B}.
At the beginning of your end step, create a 0/1 black Thrull creature token.
Breeding Pit must actually be on the battlefield at the beginning of the endstep in order for you to get a Thrull. The getting of the Thrull is not part of paying the upkeep, it is part of having the card on the battlefield.