Fen is a biome with swampy flowers, trees and mud, with muddy pools and mainly 3x3x8 or higher trees.
Mobs & NPCsEdit
Slimes can be found above ground in the Fen. Passive mobs such as Cows will spawn in Fen.
After BOP 2.0.0, Oak Trees changed to Dark Oak Trees and villages began spawn in Fen. Villages no longer spawn in Fen biomes, according to the most recent config files.