The Mesilane is a brand new dimension with new biomes!
Build a portal out of pumpkins (The same way you would build a Nether Portal with Obsidian) and light it with a Mesilane Relic, crafted with 1 Yellow Wool, 2 Black Wool, and 2 Glass Panes. Then you can enter the Mesilane dimension!
In Mesilane, you will find two new biomes (as of 20W16A): the Pumpkin Valley, and the Desolate Valley.
The Pumpkin Valley is made up of pumpkin trees, with strange stems and pumpkin chickens inhabiting the land. Pumpkin Juice makes up the majority of the rivers and lakes, and you can use pumpkins to craft Crafting Tables, Sticks, and a whole new set of tools!
In the Desolate Valley, red sands litter the land, with the stumps of a former forest serving as a stark reminder that these valleys were once home to plentiful life. Lanterns light up the chests hidden around these valleys, the chests of those who died trying to cross them...