This will be from memory and probably not well put, but Targon is some kind of celestial civilization that may or may not have started on Runeterra, but maintain some connection with/control over it. They are the ones who enslaved A Sol with the helmet he wears. The Pantheon in particular is shown giving A Sol orders in the dragon's lore.
To continue influencing Runeterra (and possibly other reasons like communicating with A Sol), Targonians can possess people who climb to the top of Mt. Targon if they find them worthy. The Targonians who possess a host have come to be known as the "Aspects". The Aspects seem to have significant differences in how they treat their hosts (a benign, beneficial relationship like Taric and the Protector vs. the Aspect of War (aka Pantheon) totally eliminating Atreus, the original host).
For some reason there are an unusually high number of Aspects active on Runeterra atm. A Sol's lore hints at dissent among the Targonians, so it's possible that the Aspects are each preparing for some kind of conflict among themselves (or, if they don't care, perhaps ignoring the conflict altogether) and working with their vessels to the extent they see fit to further their own mysterious ends. In at least one case (Leona), the Aspect bestows power and knowledge/awareness of some truth and leaves it to the host to act.
Like I said, some of this is probably inaccurate or speculation, but it's the impression I'm left with after reading A Sol, Leona, Diana, Taric and Pantheon's (RIP Atreus) lore. I am particularly unsure of the nature of Targon itself (Aliens? Ancient Runeterrans who long ago left for space? Purely spiritual celestial beings? Maybe someone else has found that out).
