This combo is a lock that can prevent the opponent(s) from ever having nontoken creatures.
- Assuming you have a 4th creature to sacrifice, and all 3 combo pieces are on the battlefield, activate Viscera Seer to sacrifice the 4th creature.
- Grave Pact triggers and forces all other players to sacrifice a creature.
- For each nontoken creature sacrificed, Ogre Slumlord triggers, exiling that creature and creating a Zombie token.
- Activate Viscera Seer again to sacrifice the tokens, triggering Grave Pact. Repeat until opponent's board is wiped.
Keep in mind the opponent(s) can just sac a token creature to Grave Pact and Ogre Slumlord won't trigger, meaning the lock stops happening. This is much harder to prevent in multiplayer games.