Under the Rocks Trophy in Lichdom: Battlemage
Under the Rocks
Maelstrom's secrets revealed
How to unlock Under the Rocks
Story Related, can't be missed.
This didn't unlock on my first play through but I'm not sure what caused the problem. A second play through unlocked it without a problem. It should unlock immediately after Surf and Turf as you approach the reliquary which appears immediately after the boss fight.