The Queen of the Abyss is the Reptids' Queen, and is praised by them like a deity. She lives in the Reptid Catacombs and is the boss in Chapter 24. She looks like a large dinosaur, and strange enough, is Fire element compared to the usual Reptids being Ice element.
In the battle against her you'll be attacked by various group of Reptids protecting their queen. She's a tough enemy; the best way to defeat her is by waiting for her to try to swallow you or one of your allies and throw one of the bombs on the floor into her mouth. You can also let her try to eat you while carrying a bomb and critically damage her.
After defeating her you'll be awarded with the Flame Reptid.