Whats a good bossmonster to tell my players that they need to leave NOW.

So I’m creating a dungeon for a magic item, my idea is that after they have retrieved it they will encounter a powerful enemy and will have to flee the dungeon (think the Balrog in LOTR). The party will be 4-5 players at around level 3, so I’m thinking about something around the CR 10-15, hopefully not with a too powerful ranged attack or to high speed so that they have the chance to flee. The creature should also have the menace factor. I understand that a large part will be me describing the encounter, but does anyone have any tips?