Blood-Queen Lana'thel, Aka, Warlock's Legendary from KFT, is not a demon, i dont play warlock alot but that's really bothers me, it would be good for warlock if he had good demons, like dragons with priest, and elementals with mage and shaman.
i hope blizzard do something familiar with demons.
Because she is a vampire, and vampires are undeads. Maybe, you are a bit confused because Mal'Ganis is considered a demon, but that is because he is a dreadlord, not a vampire (dreadlords are just demons with some vampiric abilities).
The answer is because she's not a demon, and people would be fuckin pissed if she was. Wrath of the Lich King was the most popular expansion in World of Warcraft history, which means people were really hesitant about whether Hearthstone could be true to the source. We've already seen people ask these questions on cards before, Icehowl being one, so Blizz decided to not fuck around and just make sure everything was exactly right. You can click on Blood Queen Lanathel in WoW and it won't say she's a demon, so she's not a demon in Hearthstone either.
"The San'layn are a race of elves, lead by a host of 'vampiric blood elf princes' who appear to hold influential positions within the Scourge, often acting as ambassadors, overlords, and servants in direct contact with the Lich King."
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She is not a demon because she is not a demon. Maybe let's change Illidan Stormrage to an elemental that sounds good. Unplayable anyway, trash card with a trash mechanic that does not work in a control deck. The only way discard ever worked was in Zoo and you don't want lifesteal weak stats in zoo.