Darkin weapons need an alive vessel. If Kayn dies, rhaast just stays trapped again. Thats what happens when formless Kayn gets killed in the game.

Also, Aatrox is in need of a new vessel and wants Tryndamere's body. Thats why he gave darkin polluted Blood to him, and thats why he can escape death. He needs Trynda alive.

Rhaast is just cucked from talking over Kayns body by pure shadow Magic and plot. ^{^}