The Magmar are highly in tune with the ‘worldtree’. Their mother (the golden chrysalis) was destroyed ending their reproduction. This has been given. The 12 Magmar are spiritual and most spent many of their lives meditating on the universe and the world. They were left the fuck alone by the other factions, until there was a mana shortage. Then starhorn showed up, kicking ass and taking names as the 5th ‘Vanar’.
Starhorn is definitely friendly with a lot of the other factions, seeing as he gave (I think) Kaleos his sandshield among other things. Starhorn, however, is not completely subservient to the Worldtree. The location of the second tree should have been kept a secret but Starhorn wove it into the Magmari’s Dance of Dreams (the race’s collective subconscious). This goes against the Worldtree’s commands.
Afterwards the Magmar tried to advise the Emperor but were distrusted after the emperor felt like he could not trust the Magmar. Now whether this untrust is due to the Emperor becoming unhinged or actual truth. The Magmar spread out afterwards.
From there we know very little, Vaath went to the Magmar continent and gathered all the creatures there for safety, like some kind of lizard-Noah. Starhorn wandered into Vetruvia. The Magmar leader stayed with the emperor, trying to advise him.
My own speculation:
The Magmar are in tune with the tree, they seek for balance but also want to improve the world, otherwise they wouldn’t be trying to help the Emperor and the other Aestari. They were very spiritual but something later in the lore changes them. They are still protectors of the world but became a lot more brutal. I also think they turned against the Worldtree in later days, especially if the Eyos beeing evil speculation is correct (which it seems to be so far)
Very, very far-fetched speculation:
I believe the Magmar compete for the crystals because they have found a way to revive their mother and they need crystals for that, massive amounts of crystals. That or their immortality made them selfish.