Diablo has added so much fun legacy content in Season 15, and one of those things that players has discovered is Whimsyshire! This was originally from in Diablo 3, and now comes back to us in Diablo 4. This guide will teach you all about it, from how to find it and more.
How to Access Whimsyshire in Diablo 4 Season 15
Whimsyshire Returns in Season 15!
What is Whimsyshire
Whimsyshire is a level from Diablo 3 that is full of exactly what it sounds - Whimsy. This is a level with rainbows, evil teddy bears and unicorns. This was originally in Diablo 3 as a joke, that some players have come to love. Now with the return of legacy content in Season 15, Diablo 4 players can experience it (maybe a little less vividly this time)

Complete a Waking Nightmare
A player has reported that upon completion of a Waking Nightmare, they had a portal to Whimsyshire open up. This appears to be a random occurence that can happen in the game with no way to force it.

Please stay tuned as we gather more information about this.
Liquid Rainbow
Another way to visit Whimsyshire is to find and consume Liquid Rainbow. This should open up an immediate portal into Whimsyshire that you can take. Liquid Rainbow is a consumable that can randomly drop anywhere in the world. You are also guaranteed to get one from the 5th tier on your Season Journey.

Consuming Liquid Rainbow will open up a portal where you are to Whimsyshire. Then you can blast away some unicorns and teddy bears!

Find a Goblin
There have been reports of players finding Goblins in the Helltide, and when they kill them, a portal opens up to Whimsyshire! Personally I had mine open from Murl (the Obols Goblin), still currently investigating if it can be opened by any Goblin.