Obviously, X-spells were great mana sinks, but artifacts with healthy activation costs were also big-time favorites, and Mishras Factory was a potent buy-out for those spots where you needed to Mana Drain but had nothing to use it on, plus nothing on top of your deck
Entering the battlefield tapped isnt a terrible downside, especially early in the game when we werent going to use one mana anyway
Grotesque Mutation The idea of a physical manifestation of mutation for dark paths followed is very Mythos in feeling, which makes Grotesque Mutation absolutely on theme
Ugin, the Ineffable Ugin, the Ineffable making Eldrazi cheaper amuses me since Ugin canonically opposed them and sealed them on Zendikar
Urza's Ruinous Blast This can be both an amazing exiling removal spell since only nonland legendary stuff avoids it, or bad since your foes might have a ton of stuff like their Commander, planeswalkers and classics like The One Ring too that dodge it