this post will be updated as time goes on with more info and pictures.
T3 bosses are triggered, force spawned if you will, by handing in a Cyan stone in plane of sky.
you farm the stones in HOH but you spawn the bosses in Plane of Sky (yes a different zone ...)
plane of Sky has 3 islands where bosses can spawn on, it all depends on which island is available at the time.
islands are on 1 hour refresh, so if someone spawns a boss on island 1 it locks it out for an hour. Note: if someone in their own instance spawns a boss on island 1 it also locks out the public instance, they are all interlinked.
once island 1 is locked out the next boss will spawn on the next available island once the stone is turned in.
the avatars in HOH each drop at least 1 Red gem:
they can also drop an additional gem of a different color.
the red gems are then combined, and the result combined etc etc until you get a Cyan stone:
here's the combine:
the Cyan stones can further be combined into dark blue stones (to spawn T4 bosses) and then again combined to get a purple stone (to spawn Epic 4.0 boss).
2 Cyans = Blue(dark blue)... spawns a T4 boss
2 Blues = Purple Spawns MCP (Epic 4.0 boss)
Will edit for corrections/typos etc later if need be as it's 4 AM and I'm a bit tired =-).
