Let them. Just have the ring draw them into even bigger trouble every time they do. Like he saves you from being chased by a pack of owlbears by setting the forest on fire. Now you are being hunted by a circle of pissed off druids.

You could also random or as requested use older rules for the same spell name. I.e. the fireball does volume now like from AD&D

For sure. Lots of older lore and names. Especially unhelpful things

I did something similar, also on a whim. It was an actual wizard, but a very high level one. I treated the ring as a Magic Jar, and made him very keen on getting a body, he'd been dead and alone for 50 years. I'd have the deus ex machina side of your god be possession, as well. Yours doesn't want to *keep* the body, which is cool, but in a moment of desperation, he could still force his way in for a moment, cast resilient sphere or globe of invulnerability or whatever the situation called for, drop some wisdom, and return. This would be an excellent way to prove benevolence if the party is suspicious also. Taking someone over, telling everyone "no having a body isn't my vibe" and then leaving can help sell the godhood and also shows that they don't have to worry he's trying to trick them, if needed.

This is great advice. I'll make him sleep to recover if he does something like this, so the party has to timeout on recklessness.

Another way you could do it is maybe they could feed him? Maybe if they store a spell slot in the ring, he gets some charges, like a sorcerer, and also gets a charge every day, maybe. So he can use sorcerer metamagic to rebuild his spent spells. Maybe they can only put in one a day. I dunno if i'd actually reveal this to the players mechanically, he could probably just ask for a leftover spell slot and say that it energized him like a rest. So the result is they can shortcut these naps if they wanna rely on him, an alternative way to spend their own resources. But still one they can't direct, we don't wanna just give them access to higher level spells, but they can refill their emergency savior deliberately.

That sounds super fun. I'd worry that it strays too close into a DMPC territory though, as I'd have a lot of agency of when to decide to cast what. It's a trap!

Nah it's only a trope DMPC if you overshadow the characters. If they invest resources into this guy, they will feel like they've earned it. The wizard I mentioned, I never gave him a similar mechanic, but I have an old halfling with a magical staff, in Pathfinder 2. The staff can use a few cantrips and stores books, but I set it up so he's not magical himself. If he casts a spell from the staff, he needs a caster to donate spell slots to recharge the staff, same basic idea. It was a hit at my table, they were discussing convincing him to stay with the party as a long-term companion, but he didn't overshadow them, because I didn't give him real agency over the staff. This halfling was willing to help them if they filled the staff for him, unconditionally, so the real "trigger" is one of their casters going "alright, I'll recharge the staff", and then they got to cast one of the few preselected spells from it. Framing is key, though. A boastful spell-slinging ring would absolutely make the party feel like they've just made *him* more grand. Instead, a reluctant but easily persuaded entity can say things like "Well, that went better than it did when I had a body" or "That's pretty unorthodox, but clearly, you knew what you were doing" or similar. If the party has to try to get him to do it, even a tiny amount, then it becomes part of the plan, just like any other magic item would, even though its sapient. Thats been my experience, at least. They're included, so it's not stepping on their toes. The old halfling was a guide they hired to a temple, and an excellent cook, but a retired adventurer himself, and not the "secretly level 20" kind of retired adventurer, either. He won't budge from the comforts of his wagon, but is excited to be out and about in the world. That attitude makes all the difference, and lets me drop nearly anything I want, as he is well-traveled, and can recall anything I'd like him to.

Have you ever read the Dragonlance series? One recurring character, in the books and DnD modules was Fizban the Fabulous who happens to be Bahamut, the god-king of all goodly dragons. He was basically the epitomy of what you're talking about.

Nope. Another comment mentioned it though so now it is on my radar