You get a little model of a tarrasque.

I want a lump of coal :)

You get a lump so big it crushes the earth and everyone dies.

I want a totally-random-and-not-at-all-related-to-paladins-being-my-favorite-class magic item

(Hover over the tooltip before responding, bet you can't guess what it is first)

You get a shovel called 'Avenger'. A Hole-ly Avenger.

I want Helmut_McQuack.

A dwarven cleric falls on you and proceeds to smite you, thinking you to be a demon. Or possibly a devil. He's not sure which.

I want a full-size pet tarrasque.

It destroys everything around it as it spawns, including you.

I want IRL Helmut_McQuack.

It destroy Quackity Quack Mcquacker store.

I want IRL : More Customizable charactersheets on D&D 5ed............. or fillable charactersheets for Familiars.

You get an empty treasure chest. At least it isn't a mimic.

I don't want anything.

you get something that happens to not be named anything-

I would like a tea set!

You get a sentient sword obsessed with ducks.

I want a duck.

You get a magic broomstick, you don't know what it does, but you know it's magical one way or another.

I want an extra finger.

Your eye grows an extra finger, rendering you half-blind while providing exactly zero utility from your finger.

I want a friendly pet gorgon who obeys all of my commands the way I intend for the commands to be obeyed.

You get a Gregory Gangsta instead of the Gorgon you wanted.

I need you to bring me all the desired Xmas gifts the Santa debts me from the past years.

Santa does not care what you supposedly "need" me to do for you, and thus you receive nothing.

I want a million-dollar bill.

You get a million dollar bill, however it is useless because they don't exist.

I want LEGO.

You receive one 1x1 round stud. A brown one.

I want the LEGO Death Star set.

 You get the LEGO death star set, unfortunately it is missing bag 5

I want world peace

The Christian POPE just says with an evil smooth smile :  Oh boy, sit down and be my tender kiddo.

I want to destroy all the vehicles that uses petrol or any derivated liquid from it.

You succeed and are immediately murdered by the mob of four billion people who now hate you.

I want to be able to cast far step at will.

You can, by taking a big step, as Santa cannot grant magical abilities unless you want to be his warlock/slave elf.

I want to be Santa's warlock.

He grants you powers too great for mortal comprehension, and you both implode and explode at the same time.

I want to be a level 20 wizard.