In the cosmology of White Wolf's Werewolf: The Apocalypse, the Triat is three primordial forces that shaped the cosmos, and whose fall is the background conflict that shapes the game's metaplot.

Originally, the triat consisted of the Wyld, the source of creation, pure unfettered ideas, the Weaver, who took that creation and put it into patterns, and the Wyrm, who was the force of balance that pruned the pattern, consuming dead energies. At some point in the mythical past, the Weaver gained sentience and went into overdrive, trying to turn "pattern" into the prime principle of the universe. It spun a web that entrapped the Wyrm, causing the Wyrm to go insane and begin consuming everything, including lesser entities in the cosmology, including the spirit of our own planet, Gaia. The Wyld still continues in its original function of creation, but with the other two members of the Triat in a state of dysfunction, the cosmos is disordered. In the game (and accompanying media), the Garou, or Werewolves, are trying to defend Gaia against the attacks of the Wyrm, but since they are fighting a basic element of the cosmos, they are doomed to fail: thus, The Apocalypse.

The idea of a Triat (or triad) of universal forces that fell out of balance is a simple and evocative idea. It seems like an idea that could have come from many mythologies, but I don't know the mythological/literary antecedents of the idea of the Triat. It is also not clear how much we should think of these forces as literal entities, is the Weaver really a giant spider or the Wyrm a giant...dragon/wyrm/whatever? It is left ambiguous. But it is a succinct and useful idea. I am not insane, so I understand this is a fictional idea meant to appeal to goths in the 1990s, but I also find it very applicable to real life. It almost seems like a Heideggerian idea, and I am sure there is a Get of Fenris philodox who has gotten a little too into combining Heidegger with Garou mythology. Watch out for that guy.

Also, in something that would be a constant theme and sometimes running joke in the White Wolf media, the cosmology presented here is not a universal cosmology, and the world of Vampire: The Masquerade and Mage: the Ascension doesn't have this Triat as a background concept. In fact, in one of the books, it says that Vampires or Mages that do learn about this think Werewolves are being silly or making it all up, and even that is is all a bit suspiciously Freudian, if you know what I mean. So how "true" the concept is, is really up to the players/storytellers/readers.