I have tested it in single player several times. Generally speaking, horror seed does nothing. You can cast it if you have too many blood slaves and astral-blood mages, just dont expect anything in return.
In my test game, I have cast Astral corruption and bombarded second capitol (also player-controlled, to see the results) with horror seeds for quite a some time. Then I skipped several turns and logged on to Player 2 to see what happened.
Nothing.
