So, MC spell for Umbral Demons only (which has a very situational usefulness), a mana structure (probably will add with the next update), or dwelling spell (even less useful for general gameplay), Tyrant's Retinue for heroes is a maybe, but it has a bit of an overlap with the Umbral Disciples society trait.

A spell that makes an Umbral Dwelling is an *extremely* specific thing and it also breaks the intended gameplay, so I am not sure it is even needed.