OK I got all the items i need to make the avatar appear double checked with the priestess to make sure they are right. I put them in the altar and nothing happens. so now what?

Jusat to verify, the three items yo got were.
1) the Moonblade
2) the Talisman (not just the stone)
3) the Cup.

And you were in the temple of rillifane where there is a mage, a rakashha, and I think a glabrezu?
Putting those 3 items on the altar has summoned the avatar for me every time, who will kill the three people defiling his temple if you don't kill them first.

OK. Here's the deal. It is a bug. I just had to go through the screen myself twice. If you go into the temple before you have all three items, kill everything else in there, then leave the temple to get the items, it will not work. you must be sure to stay out of the temple until you have all three artifacts.