Instead of going all in on the counter theme, I realized the nature of Marath makes her the perfect mana sink, and thus went all in on the exponential doubling of mana
Certainly Magic has created a number of horror-infused planes, like Innistrad, New Phyrexia, and Duskmourn, but letting fantasy artists loose with the card designs and color identity in black has generated quite a body of really messed up stuff
They are Vrondiss, Rage of Ancients (a Dragonborn Barbarian, leading the red/green Draconic Rage deck), Prosper, Tome-Bound (a Tiefling Warlock, helming the red/black Planar Portal deck), Sefris of the Hidden Ways (a Genasi Wizard, leading the White/Blue/Black Dungeons of Death deck), and, finally, Galea, Kindler of Hope (a Half-Elf Paladin, topping the White/Blue/Green Aura of Courage deck)
You can think of it as a 5-mana phasing effect most of the time