The Ryuguu appears in groups of two and later three in Mysterious Unison in Basement Levels 8-12.
Its normal attacks are piercing and clawing attacks on all at melee range. It tends to use several of these in swift succession and then follow them up with a Jowl Slicer, a Special Attack that is a whack on the head with its tail that can cause knockdown. If it is at a distance, it may use Grease Wheel, in which it curls into a ball and then rolls through targets in a line.
Its HP and defense are reasonably high for this stage of the game. It is very slow in moving across the field, however, and if characters are far enough away, it may simply toddle across the field and never close distance by making use of its rare Grease Wheel attack. Using distance attackers such as Beat and Viola can thus put the player at an advantage, but a decent melee team can make short work of these. It may be better, however, to just wait to battle the Astral Lieds later instead, and simply ignore these monsters.
- In the PlayStation 3 version of Eternal Sonata, the amount of EXP granted for defeating the Ryuguu was decreased by more than half, and its HP was decreased by over 85000 points.