I wanted to make Thousand-Teeth from Danger at Dunwater into a proper Dark Souls style boss fight, so I the only (not) sensible thing and overloaded it with features by mixing the sheet with a Murgrik from Tome of Beasts 2 (toned down for challenge rating) and making it into a half-black dragon. Stat block in the picture.

Considering my party of level 3 player characters (2014 rules) will be prepared for the battle (including potions of acid resistance for all of them, obtained by genociding totally-not-bullywug for their macguffin glands), how fucked are they?

Assuming they survive, I even made a tail cut weapon for one of the player characters (who's a monk/druid). I'll post it in a later comment.