What I'm suggesting is this: A Sanctum with four uniques to choose from, but you only get one. You'd start near the center of a very large sanctum, and it's rather barren enemy wise until you get to the corners. You'd be greeted with a unique monster with three minions, based on their sphere. They'd be based off the Horsemen of the Apocalypse.
War, who'd be weak to mind and body, but resistant to all the physical damage, who'd have increased speed like Vikhor, block physical, bloodthirstier, soul eater, leadership, sure slice, and slash dance.
Pestilence, immune to body, weak to light and fire resistant to dark and probably matter when it comes about, who'd have flesh rot, venemous, poison veil, infectious, leech bite, and a new passive called Absorb Essence that increases HP regen based on visible poisoned and infected characters, more than warmth does since no SP regen.
Famine, weak to body and lightning, resistant to mind, light, and dark, who'd have gluttony, oppressor, energy siphon, life siphon, consume life, and hex.
And finally, Death, immune to dark, resistant to electricity, fire, ice, and weak to light, who'd have demon's mien, reaper's mien, weigh sin, emit void, spoil touch, and snuff out.
How you gain the monster is by helping it kill the other horsemen, and it's minions will accompany you as well. You have 100 turns to beat the other three, and you'd need a well made varied party most likely, as you'd have three separate sphere's to take on. I'd be stuck between Death and Pestilence, but they'd all be good.