Quote from Exxs >>

I would let the player keep control of the character with a clear condition: that he plays the change in alignement. He can play it smart, try to hide the change to his companion, biding his time to turn on them. Then again, unless the character as the performance skill and can act, the change in his personnality should be obvious. I would then have the other PC make regular insight check opposed by the lycanthrope PC's deception. He is going to be found out, its just a mather of time. After all the other PC were probably there when he fought the lycanthrope who cursed him, and they can retrace the origin in the cursed PC attitude change. Even if he was able to deceive them, eventualy a full moon will come and he will be forced to reveal its nature. After this fight, depending on the outcom, i would only allow the cursed PC to keep his character if he was able to convince the other PC to keep on trusting him. As soon as the cursed PC is no longer part of the party i would eighter dispose of the character form the campaing(he goes away never to return, the PC can play him in another campaing if practicle) or take him on as an NPC, whatever the player wants.

 Another thing, the character is born with the curse, and doesn't initially know he even HAS lycanthropy.

Also, would vampirism or zombification work on lycanthropes (humanoid form)?

Yes, because it just says it has to be a creature in order to become a vampire spawn, it doesn't specify a type.

By the way, why do you have half of Smaug's head as your portrait?

Have you thought of having him become a Blood Hunter - order of the Lycan?

I now have a CoS problem as well. I let a PC play the UA Owlkin, he was bitten by the werewolf. Failed the CON save. I am unsure if to allow the curse or not as they are humanoid, but fey touched race. We are already calling him a Wowlf. What are the opinions, curse or not?

Quote from Atratus >>

I now have a CoS problem as well. I let a PC play the UA Owlkin, he was bitten by the werewolf. Failed the CON save. I am unsure if to allow the curse or not as they are humanoid, but fey touched race. We are already calling him a Wowlf. What are the opinions, curse or not?

 When I had PCs in CoS get infected by lycanthropy, they were NPCs while shifted and didn't get any special abilities while not shifted.

the lycanthrope can still be hurt/killed by fire, lightning, thunder, radiant, necrotic, acid, poison, cold, etc. 

still very capable of dying

Lycanthropy is good for one encounter- but difficult to use as recurring villains. Which is why I use them in all my campaigns.

With a bit of tweaking, this can be totally fine for a PC. Here are some suggestions:

• The immunities and strength boost only work while in hybrid form.
• Changing at will is a limited resource based on proficiency bonus.
• The full moon forces a hybrid transformation, and they relinquish control to the DM.
• (This one isn’t for everyone) After every transformation ends, the player must make a DC 10 flat check or incur a level of exhaustion. It’ll make them use it tactically.

There is also the Blood Hunter class which they could dip into to get their subclass: Order of the Lycan. This allows them to control their lycanthropy with certain rules in place to make it not as OP as it sounds (But it still seems pretty OP to me). 

Quote from Cade_The_Powerful >>

There is also the Blood Hunter class which they could dip into to get their subclass: Order of the Lycan. This allows them to control their lycanthropy with certain rules in place to make it not as OP as it sounds (But it still seems pretty OP to me). 

 One of my players is a Blood Hunter (OotL) and it can be challenging to play. Pretty cool though.

The Bloodhunter definitely needs some tweaks to bring it in line with the 2024 class upgrades. 

I agree with that, although Critical Role may not do that. So hopefully Wizards can pick that up and tweak it a bit.