Once you've finished Wyll's companion quest, helped him face his past, and gained enough approval, Wyll will take you to a place he remembers from childhood. As you sit on the river bank together, Wyll will ask for your hand in marriage. Players can accept the proposal, which will lead...
there is no known way to undo this, as knocking him out appears to kill him. Additionally, if the battle breaks out in camp with Astarion present but not active in your party, Astarion will join the fight automatically and kill Scratch (bad vampire!). ...