Merge with TTO:
1. Threaten him with the "Blade of the Immortal."
2. Threaten to unmake yourself with your will (min. WIS of 24).
3. Threaten him with your true name (must have used the Bronze Sphere).
4. Convince him things will be ok if you merge (min. CHA of 24).

Kill yourself:
1. Use the "Blade of the Immortal."
2. Unmake yourself with your will (min. WIS of 24).

Kill TTO:
1. Kill him by normal combat alone.
2. Resurrect one of your companions while talking to him. Pick Morte first as he is not really dead, then you can pick someone else too.
3. Resurrect all your party members by tricking him to go check the Shadows (need to have found the "Sounding Stone" in the room with the crystal).

These are the cinematics that go with each ending.

1. Merging with TTO is (merge with TTO)+(blood war)+(credits).
2. Killing TTO in combat is (TTO dying)+(blood war)+(credits).
3. Killing TTO by willing yourself out of existence or using the "Blade of the Immortal" on yourself while in dialog with TTO is (TTO dying)+(credits).