World Bosses

What mats does one need to spawn World Bosses? Whatever it is I don’t have any, and while we are on that subject, who drops whatever it is that I need?

mats are not required to spawn world bosses, except for Callagadra (superboss/celestial, don’t take fight lightly)
Calla requires celestial essence which can be found in special Korvan treasure trove (has like 40? spawn points in the FG zone, 100% spawn chance in one of them each session) or random drop, 15%? from Morgoneth in Tomb of the Heretic skeleton key dungeon

At the moment I have 6 "Celestial Essence’s. I have bumped into these what I call World Bosses. I may have their name wrong. In any event the Celestial Essences did nothing to help me spawn the bosses. All I get from these places is “Step away” or some such. I rarely ever encounter these spots out in the countryside but they are there. I’d like to fight them so as to get better armor. Any tips would be greatly appreciated. Thanks!

As Gnomish said only Callagadra needs a Celestial Essence for her to spawn. Other celestials are tied to quests that are only available in Ultimate difficulty.

Nemesis bosses will only spawn once you have Nemesis status with hostile factions and they can spawn in many different locations in the game.

there is only 1 such spot, in a fixed location
you’re probably confusing/mixing something else up, like i said, no other boss requires material to spawn, all others freely spawned each session
if you refer to superbosses then the other 5 has a quest prerequisite as Medea said, anything else is normal spawn or fixed RNG spawn upon having the right reputation.
If you’re ex referring to the “altar” in Tomb of Ateph; it’s a faction quest, doesn’t spawn a world boss or any boss of such kind