Rift Quest:Shapers End  

Enter the Final Chamber. Go to the tear and open the rift. Defeat the wave of Abominations. kain and three Elder Shapers will appear. Start fighting them and Crucia will appear, announce that your interests and hers... coincide (for now), and she attacks Kain. Leave him for now. Burn down the three Shapers and then focus on Kain. When he reaches 49% he will take no more damage. When Crucia reaches 25% she will freeze everyone (you included) and take the key. When she leaves Kain can be damaged. Kill him.