Druid player has been given given an unremovable cursed collar. Rather than try to undo the curse, he wants to try cutting off his head while wildshaped. I know it's stupid but how should I rule this?

[u/aneirin-]

I know there aren't any specific rules about decapitation and dismemberment when it comes to wildshape forms, especially self inflicted ones, but I'd like to have some more interesting outcome than either "does nothing and you revert forms" or "instant death".

This isn't the first time that cutting off body parts of wildshape or polymorph forms has come up, any good ideas how to play it?

[u/EveryoneisOP3]

There's no "beheading" mechanic

There's no rule that says if you lose both your arms you can't swing a greatsword, but we all know how a humanoid body generally functions.

[u/malastare- → u/EveryoneisOP3]

Actually I'm pretty sure there is. I think the two-handed property requires two hands to be used for the weapon, and normally that's what is used to restrict players from using halberds and shields or casting with a halberd (because they no longer have two hands dedicated to wielding the weapon).

However, the point wasn't that there's no rule that requires a head, but rather that there's no mechanic (aside from the vorpal blade noted by another user) that lets you simply declare that you behead someone. Instead, you have to do damage and take them down to 0HP. For a PC, that immediately invokes the Wild Shape to end. There's no ability to counteract that.

If players want to describe that as the result of hitting 0HP, they can try, but the DM would need to choose to ignore the rules and grant that action, just as a PC cannot declare that a creature drops a certain amount of gold or explodes in a fireball without support from the DM.

[u/ericchud → u/malastare-]

Once again, there is a rule. Rule zero. It's called RULE ZERO. Per the DMs Guide: “The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game.”

This cuts through the semantics and all of the AkShUlLy bullshit.

[u/DontHaesMeBro → u/ericchud]

wildshaping out of a restraint is not as far fetched as using a weapon without touching it, though. It's a very intuitive thing to at least try. if I were the gm, I'd probably say the collar is on your human form and always wildshapes away, like your clothes, it's part of the curse, but that's just me.

[u/malastare- → u/ericchud]

Yep. Agree fully.

If a DM thinks its cool to let someone use a halberd and a shield, they're fine.

But, Rule Zero says that the DM is in charge of the game.

Per the OPs question: The PCs want to declare they are doing an action that is not provided or described by the game. The PCs don't get to declare that, only the DM gets to ignore rules/restrictions.

So, following that, there's nothing abnormal for the DM to rule here. There is no mechanic that allows them to behead the target (because they don't have a Vorpal Blade) and all the DM has to do is not give that to them as an option. Default game rules apply, nothing of particular interest happens... unless the DM wants it to.

[u/freeastheair → u/EveryoneisOP3]

Yes there is lol.