Magebane, which horror marks you, is also good enough to KO a horror (just like it KOs anything else; nasty in a mano-a-mano fight).
Horrors have good MR, fly in storms, have decent HP, good prot, are ethereal, and have life-draining attacks. The toughest horrors have armor-negating attacks and an area-1 death attack, IIRC. They are lacking in elemental immunities, can't soak up vast amounts of damage (a Tartarian Titan SC might laugh at most of 'em, for instance... but then, mano-a-mano they laugh at most things), theoretically might die to Elfbane and Opposition.
If you want to see numerous Doom Horrors, try the following:
Cast a heavily-boosted global enchantment during Astral Corruption
...while standing under a Dome of Corruption
...while wielding the Forbidden Light
...while also wielding the Sceptre of Corruption
...and wearing a Robe of the Archmage (another horror mark can't hurt at this point, eh?)
I'd also have suggested the Gift of Kurgi, except it feebleminds you so you can't cast a global during AC.