Act 3 spoilers
Auntie Ethel is immortal while inside a silence field during her rematch. She can be brought to 0 HP and will still be active, continuing to attack her does nothing. Dropping concentration on silence or allowing her to escape the field ends the fight immediately and cutscene plays as normal. Appears to only apply to the Silence spell, does not happen when using Sussur weapons.

When using non-lethal attacks, she will drop to the ground as expected and be removed from combat, but the following cutscene will again not play until silence is dropped.

Using pommel strike while she's at 0 HP leads to her being removed from combat while she's standing and wobbling. Dropping silence kills her.

If the mushrooms are kept alive, she's knocked out and silence is dropped, the cutscene with cutting out Vanra will play but she'll run off before the dialogue and huddle in a corner while the fight continues. Killing mushrooms and Ethel at this point initiates the dialogue with Vanra.

[Linked Image from i.imgur.com]