There is a ten piece equipment set in each of the fortresses for Stone (Archmage set), Raze (Hunter's set - ranger) and Kali (Scorpion set - warrior). If you mindread those three bosses, as well as Xanlosch (in Stone's fortress), you can collect the four piece Ulthring's set.
Kali's fortress is in Rivertown gorge, in the far south east of the fjords.
I checked out each of the sets when I found them, but didn't make a note of the stats (some of the stats, shown in green, depend on the number of pieces of the set that are equipped). None of them tempted me to switch from the collection of random stuff I was using. A couple people that posted in the
Finished Divinity II ? Tell us with which stats topic used some or all of the sets, though, and at least the archmage set has been recommended as a goal for mages to get. A couple people have mentioned they like Ulthring's set since it is only 4 pieces, and allows for more flexibility in choice for other equipment.
If you teleport to the sentinel island teleporter shrine outside (someone else mentioned a door to get outside, probably leading to the same spot, but I don't recall using it), you can jump up the nearby rocks to the area above and run over far enough there are no overhanging trees, and morph into a dragon. IIRC you can also run down the path there and either get to a spot on the edge to change, or jump off and morph before you get too far into the poison gas.
The boat in the tree is above the gas, so you can transform back into a human above it, and there are also a couple other things to find around the island, in addition to finishing off the remaining attackers, left behind when you automatically returned to the battle tower when you first took it over.
You may want to read the book yourself, though, and leave the quest unfinished...