Daein is a country on the continent of Tellius during the games Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn. Daein is a heavily featured in both games, serving as Fire Emblem: Path of Radiance's main antagonist and Fire Emblem: Radiant Dawn's main protagonist.
Ashnard is the main antagonist of Fire Emblem: Path of Radiance when he attempts to rule the world and light the Fire Emblem with chaos, reawakening the Demon King. King Ashnard rose to power when he deceived his father, the 13th king of Daein, into signing a Blood Pact, which, upon the deal not being met, kills a person every day. Ashnard patiently waited until everyone in line to succeed the throne died and then personally killed his father, granting him the throne of Daein. Because of his insane tactics and inability to regret his decision to kill his father, Ashnard is often referred to as the "mad king."
Upon his rule, Ashnard then enforced anti-Laguz trends and worked notoriously on the making of his military. Ashnard held warriors high in regards to who he favored and hired the Four Riders (often called Great Riders), who he believed were the four strongest fighters in Daein. Originally, the four riders were Gawain, Tauroneo, Bryce and Lanvega, but upon the rule of Ashnard, three of four (albeit Bryce) because of his insanity. Ashnard then hired Petrine, Bertram, and the Black Knight in their place.
Ashnard slowly plotted his invasion of Crimea because of his hunger for war and power. Crimea's capitol, Melior, was close and easy to infiltrate. Crimea's laguz alliance with Gallia would also motivate his soldiers in terms of fighting the war that would come following the invasion.
 Daein Liberation
Three years after the Mad King's War, Crimea has put aside Daein to focus on their reconstruction and instead pass the sovereignty over to their allies, Begnion. Begnion rules over Daein ruthlessly, however, and treats the country without respect, working their men to the ground and treating their citizens with little-to-no decency.
This fuel sparks a rebellion originally begun by the Dawn Brigade, who originally consisted of Micaiah, Sothe, Nolan, Edward and Leonardo. These freedom-fighters fought to save Daein from the current oppresion. Micaiah soon learns of the rumored Pelleas, supposed adopted son of the late king, Ashnard, who Micaiah believes can rally the men of Daein to stand and fight for their country back from Begnion's appointed general, Jarod, who cares nothing for the country. Micaiah joins with Pelleas' Daein Liberation Army and together defeat Jarod (after apostle Sanaki receives news of how he's governing the country of Daein, much to her dismay). Pelleas is crowned the 14th king of Daein.
Shortly following his rule, Pelleas' false-advisor, Izuka, and Senator of Begnion, Lekain, trick Pelleas into signing a Blood Pact that he thinks is a Daein - Begnion peace treaty. If Pelleas does not comply with Begnion orders, the Blood Pact will slowly kill somebody each day, the number increasing with the number day it is (E.G. one person the first day, two people the second day, etc.).
When the Greil Mercenaries are hired by the Laguz Alliance to help fight in the Laguz Alliance and Begnion War because of recent news brought by Rafiel that it was in fact the Senate of Begnion who committed the Serenes Massacre, Pelleas must comply with Begnion and support them in the war. Micaiah and Sothe, generals of the Daein army and close friends to Pelleas, are shocked at Pelleas hasty move to place Daein in a war despite their current state being reconstruction. Micaiah is given no answer at first and is compelled to fight on the front lines (due to Begnion's disregard as to what happened to them in the war) without a clue as to why they're fighting Laguz.
Pelleas eventually confided in Sothe and Micaiah about the Blood Pact and believes that if the member who signed the pact is killed, the pact itself will cease to be. If the player is on his or her first play through, Pelleas will force either Micaiah or Tauroneo into killing him, only to find out by Almedha soon after that the person who ordered the treaty (in this case, Lekain) must die, not the member who signed it. On playthrough two and after Micaiah can talk Pelleas out of killing himself and he will later become a playable unit.
When Ashera forces everyone to stone in Part Four of Fire Emblem: Radiant Dawn, Daein no longer has to worry about the Blood Pact (because stone people can not die). With this, Pelleas and Micaiah team up with Ike and the Laguz Alliance after explaining their story and hope to find Lekain in the Tower of Guidance where they march towards with Micaiah's army.
In Rebirth, part one of the endgame in Fire Emblem: Radiant Dawn, Lekain and Hetzel are the bosses. Upon defeating the chapter, Micaiah will dispose of the Blood Pact and its power of Daein will be no more.
After the Tower of Guidance, Micaiah becomes the fifteenth ruler of Daein regardless of whether or not Pelleas is currently alive (he will give his status as King to her if he is).
Micaiah will take Sothe on as a husband given they have an A-support and together they will restore Daein into the prosperous country it once was. Anti-Laguz customs cease after their rule and Daein becomes a peaceful and successful nation among those in Tellius.