This is it, the endgame. First, some unfinished business.
The Old Camp
Use the Teleport to Magicians of Fire rune to access the Old Camp (you could also use a Transform into Meatbug scroll and crawl under either gate, but the rune is quicker). Wherever you go in the Old Camp every guard or shadow will attack you on sight and your armor is no match for dozens of crossbow bolts, so you may want to get in and out as efficiently as possible.
If you chose to be a Meatbug, remain as a Meatbug until you reach Gomez's castle, as the guards won't attack you in this form. If you chose the rune you'll arrive in the middle of a group of guards headed by Barthalo. You'll learn from him that they've locked up Stone the blacksmith. It shouldn't be difficult taking them out. There are more guards in the rooms downstairs. Either way, once inside the ore baron's castle you need to take Gomez out. First deal with Scar and his friend. If you take the door furthest from the throne and entice them towards you Gomez should remain seated until you're done with them.
Once they're dead (and Raven too, if you didn't get him already) confront Gomez. His sword deals massive amounts of damage, so heal up first and be sure to combo him repeatedly so he doesn't have a chance of killing you. Once he's dead grab his key. Head upstairs and go into his bedroom (or backup throne room, or whatever) and loot the chests for some permanent stat-boosting potions.
Now head to the smithy. Clean it out, because you'll be needing it to be safe for what's coming up next. Once you've dealt with most or all of the guards inside the castle head into the guardhouse, then go downstairs to the dungeons. Here you'll find Bullit playing sentry (if you didn't kill him already) as well as a guard or two. Keep going and open all the cells until you find Stone. Talk to him and tell him you're not with Gomez and he'll head on back to the smithy.
Follow Stone back to the smithy and talk to him. Be careful drawing weapons around him as he'll threaten you (this is why it's best to deal with the guards there ahead of time). Once he's back in front of his forge he'll offer to make you a variety of things. He offers to make some rings, but anything he makes can be purchased in the other two camps or found as loot, and you'll likely have the best enchantments by now anyway. Instead ask him to reinforce your armor. After a brief moment he does so. Put on the Improved Ore Armor (Weapons 100 Arrows 30 Fire 45 Magic 15) and teleport back to Xardas. From there make your way to the temple once more to take out the fifth and last of the heartless shamans.
The temple of the Sleeper
Return to the temple and find your way back to the fifth Shaman. Use URIZIEL on him (or Wave of Death if you're a mage) and loot his corpse. Along with the usual staff and sword you'll find the Breath of Death rune. Head on up the stairs and you'll face an Apocalyptic Templar. He's even tougher than the Fanatical Templars, so take care when fighting him.
Next you will find a room with fives sockets on the ground. Activate each in turn (you don't have to select the swords in your inventory); the order doesn't matter. When all five are used the door will open.
In the next room Xardas will explain more about how to defeat the Sleeper. He says that the undead Shamans' hearts bind the Sleeper to this world, and the only way to harm the hearts is to stab them with the Shamans' own swords. He will then fall asleep.
There are four doors out of this area. The two at the far side of this room lead down to the Sleeper. The other two lead to loot. Head through either and open up each of the mummies in turn to find various potions that will permanently increase your attributes. Open the chest for some healing and mana potions. Now go to the other side room and repeat the process. As soon as you have them all, drink all the stat potions to maximise your character. If you've already raised your strength and dexterity to 100 these potions should allow you to boost these stats to 170 (strength) and 150 (dexterity), and the life and mana potions will greatly help mages. Note that when you go up a level (which may happen during the upcoming battles as you still get experience for kills) your boosted health will decrease to whatever that level's normal health limit is.
Head down one of the tunnels and kill the Apocalyptic Templar you'll meet near the end (there is another in the other tunnel). Regardless of which path you take you'll pass over cracked rocks with some lava, and then end up at the throne of the Sleeper himself. Cor Kalom and his followers are here. Kalom will greet you and say that the Sleeper will now awaken (once this happens the Sleeper will throw very powerful fireballs whenever you are within his line of sight). Kalom will then attack you along with any Novices close enough to take an interest in the fight (if you want to fight him without interruption, use a ranged attack to bring him to you). Next, clear out the Novices. Lead them back to the lava if you have trouble killing them there.
You can't attack the Sleeper directly; he'll merely cast Sleep on you and then continue hurling fireballs. Instead go up to one of the five stone faces and lift it up to reveal the heart inside. This may take more than one attempt because of his interruption, but eventually you'll get it up and stab a sword into the heart. When you do this a Demon Lord will appear. This enemy is much tougher than the Lesser Demons you have taken down earlier, so you'll probably want to lead him through the doors so you can fight him without the Sleeper interfering. Unlike Lesser Demons he is very resistant to ranged weapons, so use magic or URIZIEL to defeat him. If you have trouble fighting him at close range you can take the path to the left and jump over the crack in the lava; despite having wings he can't cross this crevice, so he will float at the edge of it desperately trying to swing at you. You can then stand at a distance and use whatever runes or scrolls you have to eventually destroy him. These are the last enemies you will fight, so feel free to use anything you've been saving.
When the Demon Lord is destroyed go back and stab another heart, causing another Demon Lord to appear. If you have some sprint potions using them here will allow you to stab a heart and run off without taking as many hits from the Sleeper's fireballs. When you have repeated this process for all five hearts you will see the final cutscene and the Sleeper will be banished from Myrtana. The ending leads right on to Gothic II, the next game in the series.