In my fanfic they use a banagic wand as the vessel for the tiny dark millhorse. This is Janna's wand of course, she claimed the millhorse, and she has the affinity for dark magic to tame said millhorse, so it's hers.
This creates a tiny magic realm in place of the original. All magical spells now function at about 5% of their usual power/size, many spells failing to cast entirely, kind of like when magic was on the fritz. No invincible solarian soldiers, no gravity deletion, no timefreezing or huge warnicorn stampedes, but you could conjure up a couple tiny laser puppies no prob. Magic would be more whimsical, rather than being both a massive threat and an ex machina to solve problems. Hell even portals would be too small to be viable for transportation; so the only form of inter-dimensional travel would be physically through the realm of magic. This turns the realm of magic into a hub where all types of beings interact, and quickly brings a market. Obviously first shop to go up will be a pudding stand. But the realm eventually attracts a Goblin-dog truck and then a Questbuy to set up shop.
The realm of magic is now physically much smaller with wells much closer together and easier to find. The inert dark-green water is reactivated into purple magic, and the sky is perpetually twilight with dark purple clouds instead of white.
We did not get to see the effects of a completely purple realm, so let me postulate here. Dark magic is still magic. And just like Eclipsa's dark spells could be cast when the realm of magic was golden, light spells can still be cast despite the realm being purple. The purple water doesn't posses the golden water's healing properties, perhaps it can do something else?
Given the limited magical supply the all new millhorses are tiny and prone to mischief rather than total destruction. Magic overall comes back, but has been nerfed hard.
Well... that's the gist anyways.