Essence of Hostility
DISCLAIMER: Your Steps and Tasks may be different
Talk to Guardian Adraste near the well in Tir na mBeo, he'll tell a story about a war between Firbolgs/Celts and the Elves a long time ago, in particular one person who was so evil and killed so many elves that after he was slain, all of his belongings were slain, except a few that were so imbued with evil that they could not be destroyed and only hidden away instead. Rumors have reached him that someone has found the location of one of the items, a cursed bracer that no elf will touch.
Andraste asks you to go retrieve the item, which is found on a Rolling Stone on the east side of the lake, in the spot where the stone & rock sheeries spawn (it also says you might need to kill stone sheeries for him to appear;
The Rolling stone spawns about half way between Scarnose and the south end of the lake. (loc=43448, 33081 Silvermine Mountains). This gets you a Cursed Stone Bracer, which you're told to take back to Andraste.
Andraste gives you three options: keep the bracer as is (it's supposedly cursed, I didn't try it on so I don't know what happens if you keep it), give it to him for a small reward, or cleanse it of the curse. I chose to cleanse it, whereupon he instructed me to travel to the northeast corner of the lake to a moss-covered standing stone where ghastly siabra spawn and find the Fallen One, who spawns only at night. Again, I'm told you might need to kill ghastly siabra to get him to spawn and only spawns once per 24 hours. Possible spawn locations: 53024, 39570 OR 44600, 33000 OR 52695, 40093 (these are all very close together)
Unfortunately there are two or three standing stones where ghastly siabra spawn in that area, and the one he turned out to be at wasn't even particularly moss-covered. It ended up being the large rock about midway between the parthanan farm and the small buildings south of it.
Give the Fallen One the cursed bracer, and hill hound(s) will spawn. I only got one spawn, but the Fallen One kept telling me to kill the hounds (plural), so perhaps it's possible to have multiple mobs appear, or maybe it's just a text error. After I killed the hound, the Fallen One attacked me.
Killing the Fallen One resulted in a Stone Bracer.