To Spawn A Boss, You Must Do Dungeons In An Area (Like The Weald) Until The Progress Bar Above That Dungeon Fills Up Completely.
This Will Spawn A New Type Of Quest, The "Boss" Quest.
Picking This Quest In The Selected Region Will Put You In A Dungeon With The Boss.
You Still Have To Fight Your Way To It!
Each Boss Has Their Own Strengths And Weaknesses, And A Full List Of All Of The Bosses Can Be Found In The "Caretaker's Goals" Menu, In-Game.
Slaying One Of Them Nets You This Achievement And Marks You As Well On Your Way To Beating The Game.
Good Luck!