Rune/Shieldskin

[Lysander]

Do they block all the damage of an incoming spell, even if it greatly exceeds the amount of damage the rune blocks? Preparing to possibly eat an Ice Comet from Eldreth for the Words of Darkness Quest and I was wondering if my shitty low level 20 rune will block the entire thing, or if I will be on the hook for the rest of the 1000 damage that Shieldskin doesn't block. Or do I have to use Harmshield here and hope that I have time to feign before he starts casting again.