Are there rules for "uncontrollable lycanthropy"?

Question

One of my players wants to create a character who transforms into a werewolf uncontrollably, either at night or specifically under a full moon, and then revert back to his human state when the sun rises. But most of the rules for lycanthropy assume either the player can willing change between states, or they immediately turn permanently and under the DM's control until the curse is removed.

I can't find any mechanics for a classic werewolf. What are they? and where can I find them?

Is there anything about whether the DM should take control over the werewolf or keep player control, or perhaps a Wisdom saving throw system that gives the player brief limited control in key moments? (For instance, the werewolf is about to make a claw attack against your ally, but you might be able to make a Wisdom saving throw to prevent that from happening.)

Answer 1

There is no rule, but Monster Manual in page 206-207 gives some insight on what usually happen when a creature is inflicted by lycanthropy.

In page 207, there is a sidebar that specifically describes what usually happen when a PC is inflicted by lycanthropy. One of them is what you're looking for:

The DM is free to decide that a change in alignment places the character under DM control until the curse of lycanthropy is removed.

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A lycanthrope can either resist its curse or embrace it. By resisting the curse, a lycanthrope retains its normal alignment and personality while in humanoid form.

When you resist the curse, you retain your normal alignment and thus the control of your PC.

It lives its life as it always has, burying deep the bestial urges raging inside it.

You can imagine that there is another beast living inside you, waiting to be unleashed. Think of double personality. When the other personality comes out, you are not you, and thus you cannot control you.

However, when the full moon rises, the curse becomes too strong to resist, [...] When the moon wanes, the beast within can be controlled once again.

When the full moon comes, you cannot resist the curse, thus your alignment change, indicating that the "other you" is now awake in place of the usual you. The DM then can rule that they will control the character, instead of you, until the moon wanes.

Of course, you can ask your DM to fight control for the character, especially when you are about to tear your loved ones. Asking for WIS save seems pretty reasonable, and it will add a dramatic build up to the scene as you try very hard to fight the bloodlust.

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But most of the rules for lycanthropy assume either the player can willing change between states, or they immediately turn permanently and under the DM's control until the curse is removed.

What is written in Monster Manual is not a rule, and nowhere is stated that affected PC must turn the control permanently to the DM. The exact wording is "The DM is free to decide", so they can choose to control the character or not.

A reminder for DM that choose to introduce lycanthropy to their game:

1. Player can only control one thing: their PC.
2. Losing control of your PC is frustrating.
3. Avoid a prolonged lost control, unless your player consent for it.

Lycanthropy can be fun. Explain how lycanthropy takes away control from your PC, and how you will do it.

1. When does the player lose control of their character?
   On every full moon? Or also sometimes when they lose their temper?
2. How long does the player lose control of their character?
   Permanently until the lycanthropy is cured? Or for only five minutes, while you describe how the PC grotesquely transform and slaughter a neighbouring village?

Establish an expectation on how you use lycanthropy to form a story with your players, and they will see the curse not as a curse, but a tool to enrich your story.

As a DM, I will fast forward the scene when they lose control, briefly describe what they are doing, then describe the result.

Answer 2

No,

Werewolves are considered shapechangers in 5e and as such can use their ability freely within the restrictions of the daily limitations. The GM could simply allow the character to be placed under a curse, see the open variations that are allowed by the bestow curse spell, that has a persistent effect of being triggered by the light of a full moon. Allowing this is completely up to the discretion of the GM.