The Horrorskin Tunic is a leather chestpiece. The only place it drops is in Mathosia, the Guardian starting zone from a rare spawn. You will get this off of the Summoned Horror around (887, 1165). Here you will find an altar with two summoners, after killing the summoners the Summoned Horror will appear. Respawn time is approximately 15 to 30 minutes.
If you’re thinking of this piece you will want to remember, firstly since it’s in the starting zone you will have to create a new character but since it is also bind on pickup that character would have to be a rogue. If you’re willing to do that then you may want to go ahead and finish gathering all of the books, there are 14 that can be added to your collection so you can read at anytime, there are also five rare spawns that drop green items, a trapped man who gives you coin and a green item after you free him, and four buff shrines. The buffs are not permanent but if you’re running around looking for everything else you might as well get those also!
Full coordinates for spawn points of books, where you can find the rares, as well as a guide to the quests can all be found at Telarapedia. *** The rest of the gear that Nikayla is wearing is cleric gear and can be found in this post.