The NPC you need is Ixpacan in the Overthere, loc pos 48, neg 1072. He's down in the ravene.
"Hail, Ixpacan."
"Hmm...is there something I can help you with? I am far too [busy] to listen to your problems though so I take that back."
"Why are you far too busy?"
"It is none of your concern unless you are truely gifted in the dark art of necromancy. If so, you will have some form of proof to showme."
This is where you hand in the Necromancer skullcap to him.
"Oh, I see you are truely gifted in the dark art of necromancy. Well I will explain my dilemma to you now if you [wish to hear]"
" I wish to hear."
"I have recently found a volume on summoning a great minion from the Great Library of Charasis but I can't find all of the needed items. Being as I am one of the [sages of Cabilis], I request you go and [collect these items] for me."
"What sages of Cabilis?"
"Ah, they are all but a memory now. We used to be welcome within the city of Cabilis but our quest for greater power led to our exile. No matter now, go retrieve the items and you will be one of the chosen to walk beside greatness."
"I will collect these items."
"As you should broodling. The the first is a brittle bone, which was once used for reincarnations. The second item is a poisened soul, this is from an iksar that died a cruel and twisted death. The death was so awful, it's spirit still roams around angry. The third you will find in the burning heat. The final item is a gem of reflection. I have yet to find someone that knows how to create one. Even those fools in Cabilis probably wouldn't know. Maybe you can locate that one yourself. Bring all of these items back to me and I shall do the rest."
Ok heres what I have so far, an angered spirit can be found in Emerald Jungle, north side. It drops a poisened soul. Incalescent Rock I found as a ground spawn in Skyfire, near a dragon skeleton on an isle with a lake of lava to the east, and surround on the other sides by rivers of lava. This may be the item in the burning heat. Also, Lativ in Trakanon's Teeth drops Lativ's remains, this may be used for this quest as well. I have yet to find a lead on a gem of reflection though.