[Taqwus]

I believe somebody once set up a test case involving an SC running into astral mages spamming horror mark, and it took quite a while for the Eater of Gods to show up. Assuming that nobody's accelerating the process by sending/calling Horrors, and you're not using an item which does.

As for Astral Claw, IIRC it requires a hit to horror mark. But yes, MR is good -- and vital if Umor shows up with the 'save-or-die' attacks, or Kurgi with Blood Vengeance +7.