Battle of the Umbra Hills

The Battle of the Umbra Hills was the final battle of the Scattered War. It took place on 21 Sydenstar around or shortly after 511 PD, when the human rebels, led by a warrior named Zan Tal'Dorei, and the Elves of Syngorn fought against the human soldiers of the Kingdom of Drassig, led by King Trist Drassig, who allied themselves with fiends to aid them. Ultimately, the rebels and elves were victorious and Zan Tal'Dorei ended the Drassig bloodline, but the vast demonic bloodshed blackened the area permanently, giving the Umbra Hills their name.

Background
During the tyrannical reign of the Kingdom of Drassig by the necromancer King Neminar Drassig (who had made an infernal pact with Bane), a rebellion of men and elves continued to gain traction and bolster their ranks. One such individual was Zan Tal'Dorei, who proved herself a warrior and a leader by rallying the broken ranks of the resistance. They lured Drassig and his army to what appeared to be the Syngornian outpost, the Shifting Keep, but that turned out to be an magical illusion; there the regrouped rebel forces, led by Tal'Dorei, ambushed the king's army. The battle saw the death of King Neminar Drassig, spreading hope throughout the continent, but also cleared the way for his younger brother, Trist Drassig, to inherit the crown.

By the time that Trist Drassig claimed the throne for himself, the armies of the Kingdom of Drassig were stretched thin and people were turning against the crown throughout the lands. Lacking the brilliance and charisma of his predecessors, Drassig saw his armies slowly forced back as the rebel movement claimed victory after victory, eventually cornering the Kingdom's army at the base of the Cliffkeep Mountains, north of Westruun.

By the account in the original Tal'Dorei Campaign Setting, a desperate Trist pledged his service to Graz'zt (who had built a bastion of influence in the nearby Grey Valley) in exchange for the power to smite his foes, and Graz'zt sent his forces to fight alongside Trist's.

By the account in the Tal'Dorei Campaign Setting Reborn, it was Bane who sent fiends to fight alongside Drassig's army, and a clever demon among them gave Graz'zt's sword Graz'tchar to Trist, who wielded it in the battle. Elsewhere in the sourcebook, the sword is also referred to as the "lost blade of Neminar the Black-Fingered", who died years before the Battle of the Umbra Hills.

Battle
Zan Tal'Dorei led the rebels on a pursuit of Drassig and his soldiers to a valley in the Emerald Highlands. There, a battle broke out between the human rebels and the Elves of Syngorn against Drassig's soldiers and the sudden flood of allied fiends. The battlefield was littered with the bodies and blood of men and demons alike, but it was finished when Zan Tal'Dorei killed Trist Drassig, marking the end of the battle and the Scattered War.

Both the Critical Role: Tal'Dorei Campaign Setting and the later Tal'Dorei Campaign Setting Reborn include an adventure called "Sword of the Demon Prince" recounting that Graz'zt himself fought in this battle but was defeated and banished to his Abyssal prison, leaving his greatsword Graz'tchar behind.

The Tal'Dorei Empire
The end of the Drassig bloodline saw the Kingdom of Drassig all but collapsed. A council was formed by the leaders of the Verdant Expanse, who, seeing the people of the land accustomed to a singular leader, nominated the war hero Zan Tal'Dorei as their new ruler. Rejecting the title of king or queen, she wished to be referred to as Sovereign. Despite the protests of their new Sovereign, the council also voted to change the name of their continent after her, to "Tal'Dorei". Once power was divided equally among the council and the Sovereign, corruption and brutality was purged and peace was finally brought to the continent.

The Umbra Hills
The Emerald Highlands were renamed to the Umbra Hills after the battle. The grass and flowers that lived there grew black and burnt, and the sap of the plants native there now includes the blood of the demons that were killed during the battle.

The Arcana Pansophical
All of the realm's greatest mages fought at the Battle of the Umbra Hills, and witnessing the terrifying incursion of demons and devils, they saw that the spawn of the Betrayer Gods could return to Exandria despite the Divergence. Yurek Windkeeper, the respected high enchanter of Syngorn, brought together the most trusted and capable mages to form a private society, the Arcana Pansophical, to prevent such "surprises" in the future.

Trivia

 * The Drassigs were allied with the dwarves of Kraghammer, but no dwarves are mentioned in the recounting of the battle.
 * Though Graz'zt was banished, his sentient greatsword, Graz'tchar, was left behind on the battlefield, and through it he seeks to undermine the Tal'Dorei Council and sow chaos in the realm.
 * Demons and devils are locked in an ancient struggle called the Blood War, so it is curious for them to fight together as in this battle.