Interesting book! I'm currently playing a Warlock with a couple of Blood Demons, though I usually only use one (as the Reaper as is being talked about here) and then rely on curses and a bit of fire magic. I lay down a battleforge first turn and bring out the Demon second turn.
If you want to go for several reapers, I think it would help to have some walls, as they can fly over them anyway and it can separate a mage from his guards. Wall of Fire and Wall of Bones are both good and cheap: a wall of thorns in your back pocket gives you an opportunity if an unarmoured opponent pushes their luck. Not sure I'd bother with Wardstones, although they're good with curses: I guess it depends how many dispels your opponents tend to carry.
In terms of buffs, bear strength is great on something with Vampiric, and Mongoose agility might be worth it on your reaper, but I wouldn't necessarily bother with the other ones. There is the slight risk of the opponent just running away because your Demons don't hinder him, but if the Warlock himself is in melee that's less of an issue. Lion savagery only seems worth it if you also have cheetah speed, and that's a pretty massive investment.
I'd be tempted to replace at least one of the vampirisms with regrowth: the latter is more flexible (i.e. can be used while running away if you're injured) and against so many curses I don't see your buffs being dispelled that much anyway. My warlock sometimes has the bloodreaper's healing, regrowth belt and vampirism at the same time, which means he can soak up shocking amounts of damage and that the opponent really can't afford to pause to build resources.
I'd also be tempted to have at least area attack spell in case of swarms. If you're surrounded by 4 thunderift falcons with a rajan's fury or two down, a ring of fire is very welcome indeed.
I was going to say that Dragonscale Hauberk beats Daemonhide, but actually with bloodthirsty creatures it might be convenient to cause small amounts of damage to opposing creatures and make them tastier targets!
I would also consider the forge. The main boon is action efficiency, and a chance to react before quickcast. Given you're presumably going to spend most of your full actions hitting things, an extra action wouldn't go amiss