It allows you to pick up your Slivers which essentially makes them invulnerable to being killed by any possible effect that doesnt have split second
These are interesting because they are almost exact opposites of each other when considering which decks want to play them: DefileDefile wants to see play in faster, most likely more powerful versions of mono-black, while CorruptCorrupt has been a mainstay of black-loving durdlers everywhere since the inception of Elder Dragon Highlander
Their trigger reads, Whenever a land enters the battlefield under your control, you may gain control of target creature for as long as you control Roil Elemental. So, basically, as long as you can keep Roil Elemental alive, you can absolutely steal every other Creature on the battlefield
The deck is helmed by Firkraag, a manipulative red dragon from Baldur's Gate II
There aren't a lot of decks you build that purposely want you to be hellbrent (without cards in hand)