The intersection of Achilles Heel with Off with His Head!.
When a character or group is lauded to be immortal, indestructible, or otherwise unkillable, oftentimes one of the few if not the only things they are not immune to is decapitation. This could be a reveal, it could be known from the getgo as "the only way to kill them"; or, as in the quote above, it could be in-universe speculation which [...] [tvtropes.org]