Larian Studios Forums Divinity 2 - Ego Draconis (English) Divinity 2 - Ego Draconis Cave in

There is a hidden storage room in the cell diagonally opposite the prisoner, to the left of a cell with a busted door (check the map and it should be obvious which wall section inside is movable), where you can find a couple kegs of explosives (one to clear the cave-in and another to open a short passage a little after that).

To get to the hidden storage room, there are a couple vertical wooden beams at the back of the cell, with a button on the left beam. After that there is another button (on the right IIRC) to remove another false wall.
The two small kegs of explosives inside are on a table, IIRC, and there is a bit of loot to collect.