From Blood on the Clocktower Wiki
The Fiddler decides a winner if the game must end due to time constraints, or to a stalemate. Evil wins on ties.
"Once per game, the Demon secretly chooses an opposing player: all players choose which of these 2 players win."
There are just four players left alive. Each day, nobody nominates. Each night, the Demon chooses a dead player to kill. Since this could go on indefinitely, the Storyteller adds the Fiddler so that the game can end.
Some games will not have enough time to complete. Maybe the venue you are playing at needs to close. Maybe some players need to leave unexpectedly and the game can not continue without them. Maybe the Townsfolk refuse to kill during the day, and the Demon refuses to kill at night.
The Fiddler can be activated by you at any time. To do so, all players close their eyes while the Demon selects a living good player to challenge to a fiddle contest. Then, after a minute or two, all players will choose (by raising a hand) which of these 2 players wins. The game is then over, and the winning player's entire team wins too.