Horror spells

[Tolkien]

I've never really used Horror Seed or Dream Horror, so I'm not in a position to comment on that. Send Lesser Horror isn't too useful, Send Horror though is absolutely wonderful. Horrors lack elemental resistance, which can be a really big deal when dealing with enemy thugs or certain nations (I found that out when I spammed horrors one game against a bunch of Utgard's werewolf thugs: frost/fire brands negate quite abit of the advantages of horrors, etc), but overall, are a fairly great remote attack spell, which will consistently rout most PD or mid-level armies (although that might depend on the magic available). Plus, it uses blood slaves: you can always hunt for more blood slaves.

[Illuminated One]

Send Lesser Horror is a great way to rout PD. If one isn't enough it's cheaper to send two than a normal horror.
They also work against weaker thugs that rely on buffing quite well (Eagle Kings for instance).