Boss's Spawn randomly all over the map.
I will give some brief details on what boss's i know of and how to kill such boss's.
In alot of the quest room there is a custom boss Such as Astral planes quest, The dojo Etc.
Mind flayer spawn which is 1k+ in level, Each kill on a mind flayer you yield 1 Mind flayer point. Saving up mind flayer points you can summon a boss from the northern side of the spawn by clicking the bottom of the lamp next to the portal and running past the magic barrier. Once in this room you must start in the north west tile run south east north west south and then you have 2 seconds to make it past the next magic barrier. From here you can summon boss's.
All boss's drop Artifact power, Which helps to level up your artifact, Please see artifact page for more information
Pictures will follow in due course.