Once you have the ULU-MULU head back into the Orc Territories towards the Orc city. On your way there you'll find Ur-Shak near the city. He says that there are two ways to enter the Sleeper's temple. The first way is to use a teleport spell you will find in the Shaman's chests, but Orcs hate magic and will attack you if you use it. The other way is to climb the Sleeper's statue and there's some kind of switch there (note: you need to change into a bloodfly and fly onto one of the nearby "outer pillars" near the statue. It can be awkward to do, but it can be done. Then, change back into human form while on the small "outer pillar" and use telekinesis spell to acquire the object on top of the main "statue pillar". This statue object then works with the switch to allow access).
He'll also tell you that you need to have the ULU-MULU in hand when you cross the bridge, but it works while sheathed too. If you draw your ULU-MULU (it's a weapon really) the Orcs will say to put the weapon away, only in Orcish. Walk around and find four Orc Shamans kneeling before the Sleeper's statue. Nearby are some caves. Go in each one and take what's in the chests. One of them should give you an Orc Teleport scroll. Use it near the barred gate next to the area where the Shamans are. Once you're inside, if you go near the gate the Orc Shamans will start fireballing you through the gate, but you can easily dodge out of the way. The other way of getting past the gate is to climb the statue and use the action button and forward to open the gate. There's some kind of switch there, but teleporting is easier. Continue forward into the Sleeper's temple.
Once inside, a new journal entry will be added. This is the first time it does so without talking with someone. Keep going until you see a big entrance with the largest pitfall in front of it you'll likey ever have seen in a game (you'll fall for ten seconds, and considering how fast you go when you fall, ten seconds is a lot). Luckily there's a pillar fallen over you can walk across. Once inside, you'll fight some skeletons. The warriors are the biggest threat to you since they have the best weapons. Kill them first, as the scouts and the regular skeletons aren't going to hurt you that bad. You will also come across two or three skeleton mages inside the temple. They summon three extra skeletons to fight with you, so go after the mages as soon as you can so you save yourself a headache later.
You should be in a big room now, with three barred gates (one is down a ramp) and an open door. Don't go in yet; go in between the two gates that are closed and there's a stone block with some symbols. Remember the color pattern. Now go in the open gate back in the main room. You will come across a small hall. The door behind you will close and two skeletons are released from the cells to fight you. Kill them and collect some potions and random items in one of the cells. Look for a switch in both cells to open the gate in front of you. Now you see a big pillar in front of you. Go the sides and look for the colors that you were supposed to memorize. Go the proper color's switch and turn the switches in order. The three gates will open, and now you have access to most of the temple.