I like the thought of the Blood Demon as a Blood Reaper. I think the Dark Pact Slayer will no longer be the go-to Reaper, since this is much cheaper for only a minor loss in damage and covers the main weakness of the Reaper (no survivability boost).
I'm not sure how much the new Warlock will like this demon though. Seems she wants her minions to deal Fire damage, so she might want to bring Flaming Hellions instead.
Really? Blood Demon is only 1 less mana; I wouldn't really call that "much cheaper." And while it has Vampiric, it doesn't have any piercing, so I dont think it's going to be healing more than a couple points per round, at most. It's abilities are also heavily reliant on the enemy creature being Living, whereas the Slayer just has more flat health, armor, and dice (which also scale better with the piercing +2).
BD seems pretty ideal for picking off small living creatures though, which it could easily destroy with just one attack, rather than being the frontline tank Slayers are.
So, I really like this demon because it opens up new options for Warlocks, as well as growing the rather small Flying creature pool for every mage, but it's more niche and easier to counter than Slayers so I doubt it will replace them.