If I am looking for a specific altar, I usually look at http://www.heroiklim.com/immortals.html. Like many positive and negative immortals, Dergati seems to have just the one altar, in the Crossings' Temple.

The easiest way to get a favor from them is to carve a bead (or have another cleric carve on for you if it's too hard), clean the altar, put bead on altar, and pray a few times. You'll sacrifice the bead or a orb. You can also try others things to sacrifice, usually cards a re a good thing, and those items are also stated on Heroiklim's site.

Nikpack
player of Celeiros

Climbing List:http://www.elanthipedia.com/wiki/Climbing_skill
Swimming List:http://www.elanthipedia.com/wiki/Swimming_skill

And while I am evil, I try to avoid being just plain mean.
-Z