I don't fully understand the particulars, but from what I know, a young woman from the Seelie court was granted a gift by her Queen and this gift was a sort of stone. Celegorm was sworn to retrieve it, and so he gathered men and marched to battle.

The Unseelie side did well at first, but then great and powerful reinforcements arrived --[ Harry breaks off for a moment to get a drink of water. ]-- and the tide turned against us. Your brother negotiated a truce and exchanged his shard and life for the stone.

The woman executed him and took his shard. I didn't trust her to be ...to treat him with dignity, and went to make sure that his death would be quick and clean. It was. His brother took the body to perform the appropriate rites.