Maugrim's Sanctuary
A- To War Zone
B- Aribeth
C- Maugrim
D- The Fourth Word of Power
Aribeth is very powerful. She's a 20th-level paladin/18th-level cleric. She has a devastating assortment of spells and is immune to critical hits, death magic, fear, paralysis, and mind-affecting spells. If that weren't enough, her sword has vampiric regeneration and does additional damage against good-aligned players. The good news? She'll eventually surrender. When she does, you may be able to convince her to return to Nasher. If you have her ring it will help, but you can still persuade her to do so. If you don't, you'll have to finish her off.
The fallen Aribeth.
Maugrim is an 8th-level cleric/20th-level wizard, and he has two servants of flesh (19th-level constructs) at his side. Get in close and start hammering away--he has very little in the way of protection from melee, so the battle should be slightly easier than the previous one, at least once the servants of flesh are out of the way. When Maugrim is dead, grab the fourth Word of Power and return to Castle Never.
The Source Stone
In Castle Never, return to the dungeon and talk to Haedraline. She will tell you that you need to enter the Source Stone and kill Morag. Descend to the caverns, place the fourth Word of Power in the empty altar, then enter the Source Stone.
The Source Stone: Sanctuary
A- To Castle Never
B- Asheera
C- To Source Stone: Guardian Lair
In the first tunnel, you will meet Asheera, a strange woman who resembles Aribeth. Asheera will give you her amulet, which gives you an armor class bonus of +5 versus reptilian monsters, 50% immunity to electrical and fire damage, and a spell resistance bonus of 18. Unfortunately, as of v1.19, the item has a level requirement of 26 so you won't be able to equip it. This is a shame, because you'll face a small army of Old Ones in the Sanctuary. When you reach the center of the caverns, rest up, then use the portal to reach the Guardian Lair.
The Source Stone: Guardian Lair
A- To Source Stone: Sanctuary
B- Guardian Dragons
C- To Source Stone: Inner Sanctum
The final battle.
Not one but two dragons await you: a 26th-level corrupted copper dragon and a 28th-level corrupted silver dragon. They will both attack at once. Stay near the portal to the sanctuary if you can't take them simultaneously and do hit-and-run tactics. Once they are dead, pick up both guardian keys and unlock the gates to the north. Before you step in the final portal, rest up and save your game. There's no turning back (or resting) once you enter the inner sanctum.
The Source Stone: Inner Sanctum
A- From Source Stone: Guardian Lair
B- Morag's Chosen
C- Morag
The first room contains four of Morag's chosen (15th-level fighters/5th-level humanoids) and an Old One cleric (15th level). Though this is a tough fight, save your most powerful spells and attacks for the room ahead. When you reach the final cavern, you will face Morag (19th-level wizard). She's immune to pretty much everything, but she won't be if you take out her protectors. There are eight of them, protected by a permanent wall of blades, but ranged attacks will kill them easily. Once they are dead, attack Morag with everything you have, ignoring the minions she summons in. Once she dies, escape to the portal. And enjoy your victory.