Lexicon

Aberration: Creatures created by or related to the Daelkyr, generally deeply inhuman in nature.
Byshek: Strong metal with a purple shine found in the Byshek mountains.
Daelkyr: Emissaries from the plane of Xoriat who have invaded Eberron with their aberrations. The goblinoids of the ancient empire of Dhakaan weren't enough to fight them off and were destroyed. It's finally the orc Gatekeeper druids that sealed them into Khyber. The Daelkyr long to be freed but as long as the seals hold, the Lords of Madness can't escape. It's also the name of the language they, their creations, and other aberrations speak.
Dragonmarks: Mysterious marks of power passed down bloodlines, that enhance some abilities depending on which mark it is. Bloodlines of specific dragonmarks have formed the various Houses found in Khorvaire.
Dragonshards: Crystals imbued with magical energy that are used to power all sorts of things across the world. They come in three varieties: Eberron dragonshards glowing rosy and crimson, Khyber dragonshards glowing deep blue and dark violet, and Siberys dragonshards glowing amber and golden. They each have their own specific uses.
Eberron: Name of the realm the story takes place.
Galifar: The old kingdom composed of the five nations - Aundair, Breland, Karrnath, Thrane, and Cyre - the major power on Khorvaire before the Last War broke it apart.
Houses: Half-families, half-guilds, the powerful Houses have monopolies over different aspects of Khorvaire.
Khorvaire: The continent you're living in. The other continents are Aerenal, Argonnessen, Sarlona, and Xen'drik.
Khyber: The world below the surface, below the caverns and underground passages of the natural world. Go deep enough, and you'll reach the unnatural depths of Khyber, full of demiplanes where anything is possible, even shortcuts that lead to a different place on Eberron entirely.
Last War: After the death of King Jarot, conflicts about the succession spiraled into a war that lasted a century, broke the Kingdom of Galifar apart, and ending with the Mourning. No one won the war, but Cyre definitely lost the most.
Mourning: The day the nation of Cyre was horrifically destroyed by unknown magic. Almost all Cyrans still in the nation died, the only survivors were outside its borders.
Mournland: What remains of the nation of Cyre after the Mourning happened. It's a desolate land haunted by weird magic.
Siberys: The ring of dragonshards that circle the world.
Treaty of Thronehold: This treaty officially ended the war and recognised the following nations as sovereign states: Aundair, Breland, Darguun, the Eldeen Reaches, Karrnath, the Lhazaar Principalities, the Mror Holds, Q'barra, the Talenta Plains, Thrane, Valenar, and Zilargo. They all follow a common set of laws and maintain diplomatic relations. The Demon Wastes and Shadow Marches regions have no unified government, and Droaam has declared itself a nation but is not yet recognized by the treaty nations.