'Have ye 'eard about the beast?'

The party enter a tavern. There is the usual hustle and bustle, the grizzly bartender leaning on the counter cleaning the same glass he's been polishing for 40 years. 

He grunts as the party approach. As they pass the table in the center of the room they hear hooting and jeering coming from some rough and tough looking adventurer types. They look like they've been beaten badly, blood pooling beneath the table from the various cuts and scrapes on them. 

At this point they're starting to draw a small crowd. As you move closer the biggest, ugliest looking bandit of the lot grabs one of them by the arm and yells "ERE, BOY. AVE YE 'EARD ABOUT THE BEAST?"

After some rather theatric theatre about how the beast took his eye, leg and any other body part it could grasp,  he turns to the party one at a time and asks "what have you heard of this beast which I speaks?" 

The party then each come up with some kind of feature they've heard the beast has or something they've heard it can do.

examples - Spikes on it's back, A long tail, Horns, Razor like claws. Imagination is the absolute limit here.

It's now the job of the GM to twist these into something more menacing that they have to overcome. Spikes on the back? They're barbed and you will be snagged on them unless you roll a check to release yourself.

Poison breathe? It's got an aoe of 15ft and anyone who comes within that is poisoned and needs to find the antidote.

The limit is once again imagination!

The beast cannot be slain by traditional methods. Instead each feature named by the party has to be destroyed or nullified in order for the beast to fall. It doesn't have HP so to speak.

Spikes on it's back? Find a way to remove them, or put some kind of soft cushioning over them. 

Poison breathe? Antidotal mouthwash. 

Note: most players will likely try to just blast it with big damage to start with - so have fun with it. Let them know how hard they failed!

Additional mechanic: The guy who fought the beast and is telling you about it escaped the encounter with the deck of many things and each player can get a free pull from him.

 'we couldn't kill it, but maybe with whatever this sorcery is that we stole from it, you can!'

should the party survive this encounter, the beast allegedly has an enormous stack of treasure in it's dwelling that the party are free to loot.

This could include magic items, gold, spell scrolls etc.

And if the party can bring proof of their kill back to the tavern, well, have you heard about the reward?