Wither Outposts are the Wither equivalent to Raiders Outposts but instead of Supplies you can get Wither Dungeon Keys. The Wither Dungeon Key is for the Withers Lair Dungeon. If you want to get the Key you will have to fight Wither Minions. Wither Minions will only spawn at Night in groups of ( 1 or 2 ).
Both Wither outposts can be found when travaling on Highway 2.
There are two different Outposts. The first outpost is located along Highway two at 1012 72 -563. If you keep going on Highway two or go follow the coordinates 622 71 -1220 to get to the second outpost.