Flooded cave

 is a location in.

Points of interest
The cave itself is on an island in the Ofecchia Channel, in the far northeastern corner of the map, and at the coordinates 4° 34' S, 65° 18' E.
 * This is a large cave filled with vessels controlled by an ancient battlemage lich. Wiping them out is perfectly doable, with the best results obtained by luring them away from the lich's spires and fighting them near the entrance.
 * There is a sigil of pain in the cabinet nearest to the spire of pain, making it convenient to sneak to the cabinet first, then use the sigil to begin combat. Or, you can target the pillar with a long range attack, taking it out first before dealing with the denizens.

Loot

 * Brawler's Grimoire: Dropped by the ancient lich battlemage. One of three grimoires required for the quest The Lost Grimoires.
 * Blackwood Log in a cabinet.
 * Superb Quarterstaff, Superb Sabre, Superb Great Sword, Superb Morning Star, and several pieces of superb armor.