i got the quest to kill the beholder and his cult so i went into the main cultists housing area and killed everyone then i killed the head priest guy and the guards then the beholder came after me and i killed him so now i cant finish the quest and i have the piece of the rod from that god guy any suggestions on how i can get the quest done?

Got to find the other piece of the rod. One from the God that the cultist revolutionaries send you toward, and the other in the Beholders lair.

i saw the area on the map that says his lair but i cant get into it. i walked everyone over there but the pointer doesnt change on anything.

You seem to kill too quick. I think Tad is the only one who can let you in.
Gaal gives you rod quest - meet rebels - get rod from god - meet rebels, talk to tad in the cult complex - go down into lair, use invisibility + boots of haste to find rod (and anti-dragon halberd) - kill unseeing one - find the sneaky way through the passages to get to the stairs that lead up again - return rod to temple.
Make sure your mages have lots of fireballs!

I tried this quest too, I went down, got the rod from the god, but when I came back up, Sassar was nowhere to be found. Looked all over, even talked to Tad, but since Sassar didn't tell me that Tad was my contact, I couldn't continue, so I gave up and finished the game. One of the few bugs and quirks I found in the game, like also everytime I try to identify an arrow of piercing, the game crashes. (which really sucked, as my character was an archer with Grand Mastery in a long bow, plus 6 arrows would have really made the last 2 battles go faster)

I've completed this, remember to first humour rhe cult leader and he'll give you the quest.
You have to go down into the beholders lair via the "big hole", it's quite difficult to get down so move the pointer round a bit and it'll eventually turn into a ladder, once down there you'll find the second peice in a big maze thing.
The beholder will appear, kill it (easy) and return to the cult area. Bingo, quest complete !.