I've given an eighth level vengeance paladin in my group a nice home-brew greatsword. +1 Greatsword, +2d6 radiant damage on his Vow of Enmity plus some other stuff. He's already a wrecking ball and a boss killer so I'm looking to balance that out a bit by adding a Wild Magic table that he'll roll on when using the sword. Just like the Sorcerer table, there are a lot of random magical effects that can occur when he uses the sword. The effects are good, bad, or innocuous but none are game changing.

All I need is a good trigger for the effect. When he rolls a natural 1 on the attack roll? On the first attack after a rest he must roll percentage dice? I want it to trigger with enough regularity that the random aspect is fun (the player enjoys this sort of thing) but not so much that he's wondering if this thing is going to go off with each attack.

Any thoughts would be tremendously appreciated, thanks!

I like the idea of rolling a nat one. It depends on how often you want it to be triggered. 

If its a once a day thing, I would go for first attack percentage die or nat one. 

If you are hoping for more often, then I would say small percentage (10-30%) for any attack roll under 10. 

A greatsword does 2d6 damage, so you have 4d6 rolled when the paladin hits. Make a table of wild magic effects for that, and apply it whenever they just hit the AC of the target.

Or maybe you assign wild magic to damage sequences. Two 6 is this effect, three 6 another, stronger effect, four 6 a very strong effect and so on...

That way, you don't have to add another roll.

Thanks, Jademoonrabbit and account256. I like the damage sequence idea. . . I don't think I've ever come across that before. I really like the escalating effect it would have.

If you want it more frequent, don‘t limit it to 6s, but take 18 effects and and assign them to 2 of a kind, 3 of a kind and 4 of a kind, different effects for each number.

When they roll a natural 1 sounds fun.

Alternatively, when they roll a double for the 2d6 radiant, if you want to increase the chance significantly. Given how overpowered this weapon is at level 8, I'd go with the doubles.

I hope you're giving your other players some devastating magic items though! A level 8 paladin, with Vow of Enmity and this weapon and an assumed 18 Strength, against an AC16 opponent:

Rolls to hit with +8 and advantage, giving an 87.75% chance of hitting AC16 with each attack. On a hit they'll deal 2d6+5 Physical (11), +2d6 Radiant (sword) (7), +3d8 Radiant (2nd level Smite) (13) for a total of 32 average damage on a single non-crit attack. With a critical hit, easier to fish for with Vow of Enmity, the average damage increases to 59, with a maximum damage potential of 49 normal/103 crit. Your spell casters are going to really feel like they don't do anything if the best they can manage are 9d6 damage lightning bolts, which will probably get saved and deal 15 damage.

Thanks, Bhuraelea and Sanvael, much appreciated! And yep, Sanvael, everyone now has a pretty beastly homebrew toy. Makes for some rocket tag but it hasn't been terrible and everybody's having a great time so I'm ok with it for now.

Thanks again!