Malistaire the Undying has higher that normal Animus requirements.
To put it shortly and simply, Malistaire the Undying in the Darkmoor Graveyard has an unusually high Animus requirement for doing anything with him. Whereas every other boss creature in the game needs 20 Animus for a Summon card and 30 for a House Guest, Malistaire the Undying instead needs 30 for a Summon Card and 50 for a House Guest.
I cannot help but find this to be somewhat unreasonable, as the difficulty and length of the dungeon makes it unpleasant and time-consuming for Monstrologists like myself to collect Animus. If Malistaire the Undying's Animus requirements could be reduced to the average level, that would make it a bit more reasonable.
