So like what is the deal with summoning Daemons in 40k? It seems that all sources point towards daemons having a limited time in realspace after being summoned regardless of what happens- but then the rituals required to summon the daemon seem to far exceed any real cost. For instance, in the Black Crusade books it takes half a dozen or more sacrifices to summon a basic lesser demon- and I think that demon only sticks around for a few hours. Whats the point even? Id rather have half a dozen slaves than talk to a demon for a few hours.
And what about on the tabletop? It seems that heralds and greater demons only need a single sacrifice- is Black Crusade wrong or is the tabletop wrong? Or, does it take more to summon a lesser demon than a greater? It also begs the question that if demons time in realspace is fleeting and a few hours perhaps a day at longest, and it takes multiple sacrifices to summon a single daemon, does that mean my forces were sacrificing hundreds of people before I play a tabletop game?
It all just seems inconsistent, and my question really boils down to: What does it take to summon a daemon in 40k? How long does the daemon last in realspace? What situations would actually warrant a demonic summoning?