It effectively only costs one mana (the opportunity cost of tapping it to sacrifice it, when you couldve tapped it for mana instead), and it gets the job done
I went to a bottle signing event from my favorite rapper - Yelawolf, and he was nice enough to sign my Dark Arbok card
Coupled with Relentless, that lets the dark lord of the Sith attack for free at the end of a movement, he can be in and butchering younglings before you can say "Master Anakin what are we to do?"
Neverwinter Hydra Cards that let you roll X d6 replay that awesome feeling of casting a fireball in a D&D game and dealing, say, 7d6 damage
I noticed there weren't any with giratina for the third Sinnoh game, so I decided to jump in
Rather than potentially costing more and more as the game goes on, reaching ever higher for bigger targets, Blasphemous Act sets out with an exorbitant starting mana cost that creeps ever smaller as its use becomes increasingly needed