I want to add a negative effect on a spell: Reduce maximum magicka for a time. Interestingly, none of the four existing effects works:

1. AlchDamageMagicka will burn x mana over y seconds, and not change the maximum. The description is wrong too, because it doesn't even mention duration.

2. AlchDamageMagickaDuration will burn x mana over y seconds, and not change the maximum. Its description is spot on. It is also completely redundant with the first, because it does the same.

3. AlchDamageMagickaRate will reduce manareg, and also states so. It is working as intended.

4. AlchDamageMagickaRavage will also burn x mana over y seconds, despite saying something completely different ("Concentrated poison damages magicka by points.")

Three out of four effects are identical. Especially the last one doesn't do what it says at all. I found out that this can be fixed by correcting MagickaRavage's "Recover" Setting from False to True (just as RavageHealth), then it actually does what it is supposed to do. I'd say that is a hard to find bug in the game itself, because this effect does not do what it should.

Are you creating a potion? If not, I'd have to ask why you're using potion effects as a base.

In my mod I have a buff/debuff that increases and decreases the maximum magicka of the player. I have created my own magic effects to do this.

Truthfully I haven't fully finished testing all of the effects, so I am not 100% sure this actually works properly, but I'm pretty sure it does.

Create a magic effect.
Effect Archetype: Either Value Modifier or Peak Value Modifier. (According to the wiki, peak will not stack with other similar effects, and value modifier will.)
Casting Type: Constant Effect (So that it's an "ability type" rather than a spell type)
Delivery: Self (so it applies to the target of the magic effect)
Assoc. Item 1: Magicka

Make sure Recover is checked.
If you want it to reduce their maximum magicka, check "Detrimental", if you want it to fortify, leave it blank.

Now create a spell and add the magic effect you created. The magnitude will affect how much magicka to reduce/add.

I have now created my own base effects for magic to do this. The important part is that Recover is checked, and I think Ravage Magicka should have it checked too by default (Ravage Health does!). This is a bug in Skyrim. I'll have to play around with Constant Effect versus Fire&Forget, and Delivery.