To unlock Dempsey (Zombie Character) in Black Ops 4 Blackout, you need to play find the Juggernog Bottle, get a grenade elimination and complete the match in any place. If you fail to complete the character mission for any reason in one match, you need to find the Juggernog Bottle in the next game again.
Dempsey’s mission item can be found in the underground Diner Bunker in Nuketown
HOW TO GET THE JUGGERNOG BOTTLE IN NUKETOWN
Nuketown has multiple hatches with ladders that lead down to the underground bunker. Use the one east of Nuketown to get to the diner quickly. Once you’re down, run straight ahead and turn right and you can find the Juggernog Bottle in the center of the Diner.
To unlock Dempsey, you want to find a grenade or cluster grenade as quickly as possible down there. Grenade eliminations are a lot easier in close quarters and once you down one enemy with it the challenge is already completed. You only need to finish the game and don’t lose the Juggernog Bottle.