Homebrew Cursed Wish Species Details

Your Character was the target off an ill thought up Wish spell, the spell would be in the form of some magical item that breaks after use.

Your Character wanted to be stronger, more powerful or have unique abilities, perhaps a Fighter that Wished to have the strength of the a Giant, maybe a Wizard that craved the magical powers of a Beholder or a Rogue that thought having the abilities of a Displacer beast would be beneficial? Whatever the reason your character choose to take the power of a monster, and the wish spell worked its magic and here you are! Transformed into a hideous Giant, Beholder, Displacer beast or whichever beastie you and your DM decide on. You are still you and you can still speak the common tongue, have all your memories and desires but you take the creatures stats except you do not have any weapons or worn armour that creature normally has (so you don't have a Frost Giants great axe or patchwork armour for example) and the world around you will react to you the same way it normally would to that particular monster. 

Meeting the party: 

Perhaps you stumble on their campsite at night approaching tentatively with arms spread wide "Hello fellow travellers do not be alarmed..." or maybe they find you quietly lamenting and offer to help, maybe they where their when you used the Wish? Or they tracked you down after you stole the magical trinket that lead to your current predicament and now that the trinket is broken and no money will be forthcoming for retrieving it they think a Frost Giant would be pretty handy to have around?

The World/Game:

In most cases the world will not look kindly upon a Drake, Giant etc even if it is accompanying a band of would be Heroes, so have fun with the guards at the city gate or maybe the inn keeper will allow you to poke you're head through the window to enjoy a barrel off mead in the evening? This 'Race' can be easily power gamed and abused but is meant more for a fun RP experience and don't forget many adventures take place in the narrow halls off a dungeon how will your character take part if they can not continue into the small cavern? 

 

Cursed Wish Traits

Due to the intense Magical power it took to transform you, you get the following

Magic Resistance

You have advantage on saving throws against spells and other magical effects.

However due to fragility of your current state of mind you take double Psychic damage

Cursed

Over time you will eventually become more and more like the creature you have been turned into, bonds will remain true but you will eventually forget that you where once someone else and that this is not your true form. If you are playing a one shot think of this regression as an event that happens in a matter off hours for a short campaign maybe it happens in a matter off weeks and for longer campaigns maybe over a series of years.

This is purely a RP mechanic and you decide how your character/beast would slowly change