Originally Posted by
Vyrerus
While it's true that seemingly the souls of Haurchefant and Ysayle came back to help the WoL and Alphinaud(not Aymeric, he was a witness not a participant) pull the eyes out of Estinien, that's something I chock up to the writers wanting a symmetrical capstone moment, rather than adhering strictly to the in world soul lore, which is already wishy-washy at best. Up to that point we're already shown that despite our first blade of aether not killing Lahabrea, merely banishing him from his host body, that we could use something very much like it to straight up kill an Ascian, and we do kill one when we, with the aid of Moenbryda, kill Nabriales.
The reason Ser Zephirin uses a javelin of light in his attempt to kill the WoL, which Haurchefant thwarts, is to put us down, since the WoL is effectively immortal unless their body is completely destroyed due to The Echo. I guess we could say the writers have never 100% decided whether or not it means soul death, but I'd always considered the Ascian soul destruction context to mean that when blades of light are in play, souls die.