The forest of the undead/undead forest would be a hard biome to find. It’s definitely rare. The grass is grey colored. The trees are spindly with very few leaves, all of which are grey as well. The tree’s wood color is also a light grey color. The ambient sound could have creaking limbs, whistling wind, and the slow rustle of leaves. Hostile mobs (only undead) can always spawn in this biome even during the day because there is a fog that keeps it dark. The fog would leek outside of the biome so that it looks more natural. There are occasional ruins of villager houses that you can find. Wolves would lurk around this forest as well. Owls, which are popular on feedback, could live in this biome, sitting in the trees and watching you. Lit jack’o lanterns and pumpkins will occasionally generate in the biome. Water is extremely murky and almost impossible to see in while swimming. This biome should give a sense of classic, black and white horror. That’s why everything should be mostly grey. There could definitely be a new undead mob that lives in the undead forest. Maybe it could be a more dangerous variant of the zombie. Maybe even an undead pillager. I’m not going to go into detail about mobs though because this is about the biome. One more idea that I had is that the moon is always a blood moon (orange/red colored) when you’re in the biome but when you leave, it transitions to normal again. I hope you like my idea.