ok, as the title says, one of my players wants to be a lycanthrope. He is obsessed by the idea of being a "were-duck". He seems like the type of person who wouldn't take things too far so I'm not worried about that. However, this campaign needs to end in six months and I don't want them to spend a bunch of time dealing with lycanthrope problems. Does anyone have ideas on how I can allow my player to be this abomination, but also end my campaign in a reasonable time?

Ducks don’t have teeth, you don’t need to worry about he or she infecting anyone. 

You can always have a monster hunter kill them if they take things too far. I doubt your players would expect foul play.

Slow down the in game timeline, so there’s only 1-2 full moons before you end. Just enough to let them do it a little, but not so it would take over.

Ways to get around it:

- Fool moon conveniently happens after the campaign ends (or during downtime weeks that get skipped over).
- The PC inherits a strain of lycanthropy that can be fully controlled.
- Accidental lycanthropy infection can be cured with a simple trip to the temple and payment of silver.
- Everyone else in the party falls into a vat of lycanthropy antidote and gains permanent immunity.

Have the were-duck form have some of the features of the Beast Barbarian, but add vulnerability to silver, and resistance to non-magical piercing, bludgeoning, and slashing damage from non-magical weapons while in beast form.

You could also have the were-duck when transformed have to save against going feral when it's below half-health, and if it fails the save, it attacks the nearest living thing.

Those would make the character feel like a lycanthrope, but limit the player's ability to derail the current story.

<harsh>The character transforms, kills a villager, the town guard hunts the character down and stabs them to death with silver spears, the player creates a new character and learns a lesson.</harsh>

But seriously folks...

Why does the player want their character to be a lycanthrope? The answer to that will affect your strategy.

Personally, I struggle to understand why a player actually wants their character to get a curse. As a GM, when I curse characters the players are all like "ARRGGHH DISPEL MAGIC REMOVE CURSE UNFAIR GM ARRGGHH!" But if there is a werewolf on the board…

I actually have two players deliberately baiting them in an attempt to get cursed. One of them was cursed, and has subsequently killed a farmer. Tonight's session is going to deal with the aftermath…

I mean, if they want to be a were duck is probably a joke. Small things like giving them a flaw where hey quack at inappropriate times and have a chance to wake up in the morning having layed a duck egg is probably plenty. Small funny things to let them run their joke with. 

As for why it happens it can be a curse from some one who wants revenge or you can have them attacked by a were duck , what ever.  I you need a story reason why it's kind of an impotent lycanthrope curse, well just say the person who cast it did a bad job. 

they are making a this as a back-up character if their current one dies so I may never have to deal with it. they thought it would be funny/cool to be a were-duck

When I wanted to play a lycanthrope I created a Circle of the Moon Druid. That gave the flavor without actually being cursed. 

This is a great idea and one I'll probably use myself, but when I proposed it to him he told me and I quote "nah, druids are hippies and they're basic. Besides, why have a class that grants you that, when it could be a race, then you could have a really cool class and be the total package.

while there are many flaws with his logic, he's stubborn and I don't care enough to stop him.

I know I am really late to this thread and your game is long over, but this is for anyone who may have an issue like this in the future and comes here.

First, I agree with Solarsyphon, this has to be a gag, so run with it.  Let it be fun.  Unless you run a more serious table, then don't, of course.

Second, here's an idea.  You could do a Ladyhawke/Shrek sort of thing where, instead of moon phases, the change occurs at night, or day...every night or day.

Third, becasue I am already enumerating.  We had a similar situation with a character who was a story arc unto himself.  This character, while cooperating with the party on all party goals, was also hunting a mysterious foe.  The target of the PC's wrath had murdered someone in his family (or whole family, or dog, I don't remember exactly)  The PC, by the way, did not imbibe alcohol, when everyone else was ordering ale or Meade, he ordered milk.  Somehow, every town we entered had been visited by this villain only the night before.  We took these characters from 5th to 11th level before the PC discovered that the enemy was himself.  The murder was actually an accident that happened while the PC was drunk and he had no memory of it.  Instead, his Psyche spit and one of his personalities was trying to stay one step ahead of the other.  The DM totally handles that with stories from barkeeps and stable boys about the dark knight who passed through the night before, drinking, carousing, and causing mayhem.

Use the Dhampir race as your basis and just reflavour it all as a Duck thing. Since you don't want the campaign to be all about were-duck nonsense, just allow them to go all beaky when they want to, but to be fully in control of their foulculties.