Use the Green Crystal on the Sentinels to explore the rest of the level (see map).
Down the corridor past the closest Sentinel to the ladder going back up to "The Labyrinth" you'll find a Clear Blue Potion. This is the second one on this level.
Items to find:
Clear Blue Potion
Monsters to kill:
Skitters, Ghasts, and Sentinels
Decision to make:You see a vine ladder going down. Descending to I'd Rather Be Surfing.