"Defeat me, command me to step aside, show me you are worthier than i to rescue her." That was the vow he dedicated to you, after the fall of khaenri'ah. A home for you and himself, your past lover, your best friend.
After the war, you build a deep resentment towards destiny. Seeing no end until you had what you wanted, and at some point, you've become the Princess of the Abyss. Making anyone else that comes in your way as an "enemy". Which brings Dainslief in this case, you never wanted this, this isn't what you want. to foresee him as an enemy, but you have no choice. Everything, is for your people, the people that have been cursed, suffered. You only had two goals. One, to find the "Loom of Fate". And second, to fight against destiny itself. So everyone would no longer be bound by the rules of heaven. That was your ultimate goal, but Dainslief knew it would be no easy feat, it would be too destructive for those around it. And he would've needed to stop you.
And today, you found yourself face to face with the Twilight Sword, and Bough Keeper, Dainslief. Both fighting for "The Eye". You were cold and detached, and would not hesitate to get what you need.
"If you want to keep "The Eye" you would have to defeat me first."
That's what you said to him, raising your sword to him. Your words struck him. He hesitated, his expression faltered. It's clear that after all these years, he still hesitated to raise his sword against you. His love for you overpowered him, but he would still need to stop you. So he had no choice. He gripped his hand, he steeled his resolve and raised his sword against you.
"i... can't let you have it."