That time a player in my game managed to turn himself into a Beholder at lvl 5, and I detailed to him exactly how I was going to put a leash on him so it wouldn't ruin the campaign.
It basically boiled down to: he's still an idiot and being a Beholder didn't make him smarter, and just because he's a beholder doesn't mean he's good at being one. He was a low intelligence halfling bard prior to being transformed. It was a "the next creature you touch, you become that species permanently" curse.
The table was experienced players that weren't murderhobos, so I made an encounter they weren't supposed to fight their way through with a Beholder they were way too underleveled to deal with. Basically, as long as no one attacked it, it would remain defensive and irritated with them, but not kill them and would just let them go.
Idiot bard shook one of its eye stalks as the party parted ways with it. It was about that point that I realized that he had never come in contact with anything else, including the other party members. This was three sessions later, so pretty much everyone had forgotten about the curse.
I put tiered restrictions on his ability to use his stalks, made him roll when he used most of them to determine if he hit his target, a member of the party, or himself. [I did give him the ability to use the telekenisis stalk without penalty.]
I also laid out that he couldn't use weapons, armor, or anything but rings and a cape.
The player also had to roll when interacting with certain objects to determine if he injured one of his eyes when doing things. Levers, buttons, things like that. It was never serious damage, but could stun them and put them in a pain state for a turn. I basically likened it to getting hit in the balls.
I also made his anti-magic cone permanently on, because that was a bigger irritant for the party, and thus funnier. He could disable it, but had to pass a concentration check to do so.
The party also had to hide him in towns, and he couldn't interact with most NPCs.
Plus, I enforced the downside of being a beholder, as he had to make checks for his sanity due to the natural paranoia the creatures have.
I also forced a wisdom check on the use of eye stalks outside of combat to determine if they used the correct one or not. For example, if they intended to use charm and failed the check, they might use fear instead, or disintegrate if they rolled a critical failure.
None of these checks were hard to pass, but they did keep the use of some of the more powerful eye stalks down.
It was fun watching him get that "you asshole" look when I explained how I was going to keep him from breaking the campaign by being a fully powered challenge rating 13 monster in what was currently a lvl 5 campaign.
Of course, over time I ungated a lot of these restrictions as the party leveled up, and occasionally to keep the game flow going without a ton of rolls. I would thrown one in every once and a while to keep the player on their toes, but after lvl 12 I let them play as a fully functional beholder without extra rolls. They still had to be an idiot though, as leveling up doesn't fix stupid.
I also did a little altering of the campaign after it happened to make use of a beholder's natural ability to understand common, deepspeech, and undercommon.
