This is the other half of Josh's previous skill; the cosmic knowledge and power he received in exchange for his left hand, which allowed him to sink Atlantis into the sea, end the reign of the Elder Races, and ensure the survival and rise of mankind. It makes him, in his own words, both more and less than human, mightier and -- eventually -- more ancient than the greatest of the Great Elders, the Archons, or the Earthlords, with all their accumulated learning and strength. Basically, this is the god of all knowledge skills.

Although it does a pretty convincing impression of omniscience, it isn't, exactly; the knowledge it grants doesn't extend to the actions of individuals, or the art or culture of societies. I'm interpreting the "hundreds of millenia of history and learning" as a comprehensive understanding of how the universe works, plus the accumulated magical and scientific knowledge of the previous civilizations of the world. He doesn't know Archon poets or Earthlord painters, but he knows how their magic and technology worked (in some cases, pretty much the same thing). In theory, he can use this power to look at the spells making Aather what they are; in practice, I'm assuming that the power cap and the nature of the spells in question make that impossible (unless someone modly wants to play with that; default is off, though). It's mostly not a personal level skill, though, unless you count the benefits of suddenly knowing everything about his world's magic and technology.

Josh gets a couple of different things from this skill. The first and most obvious, other than the rush of incredibly esoteric cosmic knowledge, is the understanding of how to use all his magical skills to their fullest extent. I'll note the specific skills affected later in this longass essay, but in general, he'll be able to apply more finesse and creativity to all his skills, spending less buck for more bang. The esoteric cosmic knowledge is also a thing, of course; it's safe to assume he knows most of the hard sciences pretty cold, as well as how to apply magic to them in various ways he can't actually accomplish yet.

He also gains the somewhat murky ability to see causal chains, if he tries; Marethyu says that the ability to see past coincidence to the reasons for everything happening is part of his gift/curse. (He calls it a curse, but after countless millenia he's kind of emo.) Mostly this will manifest in the ability to jump to accurate conclusions more often, and work from small clues to get large results. (Obviously, where this intersects with other characters' mastermind plots, we'll work things out OOCly.) He's in tune enough with the structure of the world to do certain types of magical work, like speaking the First Language if Sieh or Naha or Tempa were interested in teaching him. As a side benefit, he has an eidetic memory.

Finally, his MAGIC stat is now maxed. He's probably the most powerful spellcaster his world has ever seen; certainly in the top few.

Skills he already has that will be affected:

Fire Magic: He'll know how to spark a hydrogen fusion reaction, and also that trying will kill him in Aather. (He could've done it outside of Aather, though.) In general, he'll be able to shift the properties of fire more, pushing the bounds of the possible; stuff like setting fire to water or stone without the ridiculous lengths he's currently going to. He'll be able to move heat around instead of creating it, convert it to electricity and back, and generally have a ton more finesse. He might or might not be able to mess with nerve impulses, especially if he can embed an aura blade in someone's body, but I honestly don't know how I'll run with that if it comes up.

Air Magic: He's already got a lot of finesse with this skill, but it'll increase; he won't need his trigger, and he'll be able to do finer dexterity and control of air, actually picking things up or acting at a distance. He'll also have better control of fog, mist and other weather manifestations, and significantly better illusions.

Water Magic: Again, more finesse. Also he'll work out that he can bloodbend or dehydrate things, shift the state of water more easily, use microstreams as cutting implements, et cetera. Water's not his strongest element, so I don't have to get that creative. Some minor transmutation stuff goes here as well.

Aura Shaping, Aura Enhancement: There's not really enough to these skills to enhance; the main thing he'll be able to do is combine them with other skills with more finesse and grace. He might get more of a hint about what people's auras mean, but I'm capping that unless it's interesting. He'll be able to "study" magic a lot more in general with aurasight.

Wards: This is the interesting one; Josh now knows the Words of Power and Symbols of Binding the ancient Archons used to defend their most potent treasures or confine their most dangerous enemies. If drawn correctly and placed in the right pattern, these symbols can imprison Great Elders, most of whom were worshipped as gods; lesser creatures, like Next Generation elders, aren't strong enough to bear the presence of the Words and die slowly as they're bound. Weapons marked with certain of these symbols gain a massive increase in lethality, often transmuting their victims as they kill. He'll probably also be able to attach spell effects to wards, but the Words of Power are the big takeaway.