In the Rogue-like terminal game UMoria, the Balrog is the strongest and only unique monster in the game. He appears deep in the dungeon, and appears as a dreaded capital 'B'. Upon killing him, you are declared the winner.
In the more-complex Rogue-based game, Angband, there are multiple unique Balrogs, none of which are the final enemy. (The ultimate goal of Angband is to defeat Morgoth himself).