Well, to be fair, the grell isn't really meant to drive you insane so much as it's meant to paralyze you and slash you to pieces.
But come on, that thing shouldn't be! It hovers in the air despite no apparent means to do so! It's a giant lump of flesh and tentacles with no eyes, ears or nose! And yet, it hunts you even it shouldn't be able to even find you ! HOW DOES IT FIND YOU?!