Come, come, we're making more assumptions here.

If you look back on my last few posts, Sorin has been gathering strength for quite some time, casting small spells that actually increase his power, meaning to snowball into a powerful effect such as this one. Summoning vampires and buffing allies are both +1 abilities for Sorin. Considering that he's been staying to the sidelines and defending himself, thereby taking very little damage, he actually has a decent amount of mana to work with. Also, you're exaggerating Elbrus' casting cost. Granted, it's the most mana-expensive equipment in the game, but that said in terms of artifacts and other cards in general, it's only sort of moderately-expensive. Seven mana really isn't all that much.

You're also leaping to conclusions. By denouncing that the flavor of a demon being released through a blood sacrifice does not specifically require a spellcaster's blood, I am not insinuating that any effect can apply to any target, as long as it's convenient for me. What I'm trying to do works thematically and dramatically, and so I'm working around the game-balancing elements of the card. And again, please don't make assumptions about me as a player; what gives you the slightest reason to believe that I would just summon "everything and anything" on a whim? If you'll look WAYYYY back, you can see that I'm already working within the bounds of the RP set for me, as mind control (something Sorin does quite casually and frequently) is something I have yet to use in the RP.

That last note is a matter of subjectivity. I plan on RPing out Sorin struggling to control the demon, should it be summoned. Not to mention, that we are not literally unleashing Withengar, here. Sorin is summoning a copy of the legendary demon. At what point is a summon ever out of a player's control without another player taking control of it or its actions?