How Do I Prevent Alistair From Dying in the End of "Dragon Age: Origins"?

In the game "Dragon Age: Origins," powerful guardians called Grey Wardens have kept the world safe from demons for thousands of years. Now, an Archdemon and his armies threaten to destroy your kingdom. In the final battle, a Grey Warden must sacrifice his life to slay the Archdemon. Only your character and two of your companions, Alistair and Loghain, are Grey Wardens. Your sorceress ally, Morrigan, suggests an alternate plan. If you transfer the Archdemon essence to her unborn child, no Grey Wardens have to die. If you want Alistair to live, sacrifice yourself or Loghain instead, or agree to Morrigan's plan.

Complete the first part of the final battle at Castle Redcliffe. To do this, kill the monsters in the courtyard with the help of your team and the castle guards.

Enter the castle. Go upstairs and enter Riordan's chamber. A man in the room says you must sacrifice a Grey Warden to destroy the Archdemon.

Go to your room in the castle and talk to Morrigan. She explains that you don't have to sacrifice a Grey Warden if you follow her plan. If your character is male, agree to the plan to save yourself, Alistair and Loghain.

Talk to Loghain or Alistair if your character is female. Convince one of them to spend the night with Morrigan to save the Grey Wardens. If you cannot convince them or you disagree with Morrigan's plan, she leaves the team permanently. After making your choice, you travel to Denerim instantly.

Kill the monsters that attack Denerim. Make your way to the fort and get on the roof to face the Archdemon. Attack the Archdemon until its health is very low.

Sacrifice the desired Grey Warden if you did not agree to Morrigan's plan. Use the sacrificial Grey Warden to deliver the killing blow. For example, if you want to save Alistair, either you or Loghain must kill the Archdemon.