It's an EXTREMELY rare finding! But if you are lucky enough to spot one and get its seeds.. You can now harvest wither skulls! :D They take a while to grow but each shrub can produce up to 4 skulls!
You can also find Wither Shrub Seeds in Undertaker Chests with a 0,4% chance.
Harvest the skulls and wait for more to grow :D
This then grows to this:
DON'T HARVEST THE STEM! They work like melon or pumpkin seeds :3