Actually, given what Overlord made the Horrors out to be, and the fact that he has turned to them now, suggest his power is waning, almost as if the end is in sight. So I would say the horrors are either the last, or second last wave of Overlords forces. If this is the type of invasion which is decided by military might and not some deus ex machina, then humans are close to pushing back daimons.
I was surprised by the message Overlord sent introducing the horrors. Very out of character for him to praise our efforts, even so slightly as he did.
