So something that not a lot of people have been talking about is the essence masteries, where they are from, what they represent. But I think that if I don't have the answer, I think I have the closest thing we can get to one with all the evidence that has been given. Thusfar the story has been infuriatingly vague with the exact nature of what masteries are or where they come from, saying only that they are a malicious spirit energy that is intrinsic to the various minions we have thusfar encountered. When we harvest their magic we transform it into a 'vitreous' essence that we can transform into weapons against essences of the opposite type. However I think I know the source of each of these essences, and it all goes back to the Norn exodus from the Far Shiverpeaks, because each one of these essences is a corrupted spirit of the wild.
This ability is showcased with Raven and the High Shaman, who uses his magic to attempt to subvert Ravens power and create the Icebrood Constructs that we fight, I believe that when the Spirits of the Wild were overwhelmed by Jormag each one of them was fashioned into a distinct weapon save for Owl, who is the only one of the four confirmed to be dead.
First is the aspect of Resilience, associated with the Fallen, though not merely because they have turned to worship of the Dragon. At a distance the Fallen appear to be an army of Kodan and transformed Norn Shaman, but if you look closely you'll realize that each and every one of them is a transparent ghost. This is because I believe that Jormag used the power of Dolyak to keep their spirits in the mortal world, persisting in the mortal plane despite their death. This is why the Fallen use so many portals to bring their soldiers in and why they inhabit decrepit ruins. They have no need of a place to feed or house their troops, only a place to haunt, and all those places also happen to have a Dolyak shrine near them. One in the Fallen Mountains, one near a Raven Lock near the Ice Caves, and a third with the Fallen Kodan in the Sanctum strike mission. Not only that but the vigil investigator who keeps talking to her Dolyak while endlessly collecting evidence around her keep? When you assist her, you realize the areas she's searching through is attracting swarms of Fallen.
Second is the aspect of Valor, attached to the legions of demons, wisps, haunts, and otherworldly abominations that inhabit the forests nearby. Within that forest is a single statue to Wolverine above the ground, and each of the essences resemble a Wolverine, in much the same way that Resilience each resembles a Dolyak. Where Resilience was perverted into undeath, Valor was turned into a manifestation of fear, taking Wolverines violent and destructive aspects to their logical extreme to craft monstrosities like the Boneskinner. Thw legend of the Boneskinner as told by the Norn is very similar to Wolverine feeding patterns. They're fond of carrion and scavenging, sometimes following other predators to take their kills in much the same way that the Boneskinner mops up in the aftermath of the other kills, hunting and snatching whatever prey remains in the area. Wolverines are also known to be gluttonous, consuming vast quantities of food to sate themselves, but this is believed to be an adaption to food scarcity in much the same way that Boneskinners are created from cannibalism brought about by starvation.
Eagle is the most difficult one for me to pin down the specifics on. The essences of Vigilance look like it of course, and in the High Shaman fight you can see those Eagle statues arranged around them on the central platform. If I had to guess as to why exactly Vigilance has become tied to the Svanir, it's most likely because the blind fanaticism and zealotry their exhibit is being inverted by that particular spirit and fed into Jormags mortal followers in order to enhance it's persuasive answers, turning them into a loyal and unquestioning army to persecute Jormags wars on the other races.
I think this theory is pretty solid, and explains a lot about why exactly the Essences have such contrary effects and powers to their names, thoughts?