The Tavern

We have max hp decay, we have stat debilitation, so why doesn't the same mechanic exist for mana? It could be placed on something exclusive to a few endgame areas - maybe as a greater mummy death curse and a hell effect. Some sort of moth (as in Perdido Street Station) would be a solid candidate as well.

As far as reversing the effect, it ought to be analogous to HP rot: one point for curing, several for a potion of magic. This would make potions of magic more interesting as they would now have value as both strategic and tactical resources.

Ever play a deep dwarf or a mummy? They both have a mechanic that involves this.

I think in general we should be moving in the opposite direction and be making rotting less common, not more.

The mechanic already exists in the code base, so it would be easy enough to implement.

I don't think just adding mana rot exactly like hp rot is a good idea. We already have enough monsters specifically targeting casters (eyes of draining, antimagic, ghost moths, etc.). I would prefer to use the mechanic in a more interesting way. I toyed with a god proposal that would have used this "mana rot" as a cost for their Invocations, but I never came up with enough content.

That's permeant MP loss. This proposal is talking about something like rot where you can get the loss of max MP back.

You're asking the wrong question here. Why should it exists? Not for the sake of symmetry, that's for sure. Players often suggest that foo should be like bar because it creates a nice symmetry that makes them comfortable, but in reality, it often only reduces differentiation.