Mana-theft mechanic? (5e, Arcana skill)

[Reosoul]

I'm creating a High-Magic campaign where magic is considered 'volatile'. It's not convenient, it's difficult to control, and the stronger you become at wielding magic, the more stress it places on your mind(more credence to the Mad Wizard trope). Also, the mana(spell slots) used to cast magic isn't owned by anyone. You can sap other lifeforms of their mana and restore your spell slots, if you like. If you're empty on spell slots, you can still cast more, but have to make constitution saves as a result, and will walk away with exhaustion, even if you pass(worse things happen if you fail).

So, an important thing, is I'm wanting magic and mana to have that different feeling. I'm not worried about the idea of a player deciding to roll up an evil character and just cart around a bunch of slaves or henchman for extra spell slots(I have ways for dealing with this), but I like the elegance and simplicity of 5th edition.

So, how do I make a mechanic based around people being able to mana-drain each other?

Initial thought was that a creature can take an action to begin a contest with another nearby creature(30 ft?). Both creatures roll Intelligence(Arcana) checks, and the loser of the contest having a spell slot stolen for every 5 that they lost the contest by(minimum 1).

I'd like some feedback on the above interaction/mechanic. Specifically on how this could possibly be refined to be made more elegant and not bog down play.

Thank you, Playgrounders!

[JeenLeen]

As a mechanic, it seems pretty straight-forward and fast: a simple contested skill check. This does make proficiency with Arcana, and moreso Expertise with it, rather important, but, well, I reckon most PCs would be proficient given your setting, so that evens out.
If you wanted to speed it up, give each character a Passive Arcana rating of 10 + their Arcana rank (just like Passive Perception). Instead of a contested roll, the person draining must roll Arcana against a DC/AC of the target's passive Arcana. That removes 1 roll from the process, speeding it up slightly.

As for how effective this mechanic is, I don't have much comment. It requiring the main action means draining slaves in the middle of combat isn't efficient in an action economy sense, but it doesn't prevent an evil PC from draining prisoners/slaves in-between fights (or even good PCs draining evil folk they've imprisoned. Effectively, every break can become a long rest if there are sufficient prisoners.) But this is a powerful debuff (?), since it drains spell slots, so it seems a worthwhile use of an action, especially against stronger foes.

I'd recommend the range being 5 feet. I would recommend it requiring an attack roll (either Strength, Dexterity, or casting stat, player's choice), but that means two rolls, which increases time and makes it a less likely to succeed ability. I think both of those would make it less likely to be used on one's foes in combat. But a 5 foot range means you can't use it on slaves at a nearby but sorta safe distance.

I'm assuming this is a standard action that any trained person (thus, every PC) can do, that is, it is open to all classes. ...just, I guess useless to non-casters in the recovery sense (still useful in the depower sense.)
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Also, this may suffer some abuse with stuff like sorcerer sorcery points and perhaps burning spell slots for things like Smite, or warlocks who have quick spell slot recovery. But maybe not.