[Aleph]

Nick - It seems to be like getting lost in the void - they just pop out wherever. Certainly a bunch have shown up in places I've never cast any spells from. If they show up over a fort of yours they will start sieging it - the first time it happened to me I was like "what paratroopers could have eaten through my PD" and was stunned to see that Kurgi was sieging a fort of mine.

Baalz - I've used Horror Seed a bit but, as an SP player, I don't get feedback on how it works - and, like you, I'm using it in conjunction with Astral Corruption, so I don't know where all the horror marks on my enemies are coming from. The only thing I've noticed is that sometimes after using it extensively I'll be fighting a big enemy army and sometimes after I kill someone a Horror will pop out. Whoops! Seems much less effective than a nice mix of Infernal Disease and Send Horror, overall, and it's usually only my pretender who can cast it, so it's a low priority for me compared to the other nastiness he can be throwing around.

I've also noticed what I believe you refer to in your second paragraph - I'm pretty sure I've cast massive 100+ gem spells and only been attacked by a regular Horror, which would imply that the Doom Horrors are all busy elsewhere. If so, you're weakening your hold on the world through the Claws/Prison combo, but it's still a small price to pay to be able to continue normal spellcasting and forging. Even a regular Horror is good enough to wipe out most commanders (as the OP is so stricken by).

Does anyone know the timing on Astral Corruption? That is, if you throw a Dispel at it, will you be attacked by a horror if the Corruption is dispelled? Not that there's much chance of any blood spell being dispelled unless you're in a large MP game and people gang up to dispel it, given the prevalence of blood slaves compared to most other gem types.