Whimsydale

Whimsydale is a unique location in Diablo III that can only be accessed by killing a Rainbow Goblin. It uses the Whimsyshire tileset, and is overall identical to it. This location was added in patch 2.1.

This zone can spawn all the same Unique Monsters as Whimsyshire, with the exception of Sir William. At least one will spawn every time.

There is no special or unique reward for beating it, except for a chance of obtaining Cosmic Wings from the rare spawn Princess Lillian. It's worth noting that although it is possible to find more than one Rainbow Goblin in a game, it will open a portal to the same Whimsydale, so there is no benefit to continuing to hunt for Rainbow Goblins after one has been located. No Treasure Goblins will spawn in Whimsydale.

As of March 2017, Whimsydale container drops have been greatly increased, growing even further with difficulty. Enemy drops are consistent with normal drops of that difficulty. A single run of Whimsydale can bring rewards comparable to multiple Greater Rifts and The Vault combined.
 * Piñata: multiple Death's Breath and Forgotten Soul
 * Lovely Gifts: dozens of items, including multiple Magic Items, Rare Items and Legendary Items
 * Funny Shrooms: multiple Death's Breath
 * Pot o' Gold: large amount of gold, a single highest quality gem (Imperial at level 70)
 * Happy Cloud: several gems of highest quality (Imperial at level 70) and sometimes a pile of gold