The Daemon Wars refers collectively to two major wars between Daemons and the peoples of Idryllia. In terms of the Western Isles, the Daemon Wars never ceased, and Mariners and other folk of the archipelago often don't make a distinction between the two.
Although certainly wars involving Daemons, Dessecar's two attempts at conquering Delwynvale, and the Revenant Crisis in the Age of Reclamation are not referred to as The Daemon Wars.
First Daemon War[]
The first Daemon War would erupt in the Age of Knowledge. In service to the Dark Source, Arcane Society Archmage Tyrius Gadril would open a portal to Syn'Tarus, the Ether Pocket containing the Daemons, in a staged “accident” while opening the Society's portal in the Western Isles.
The subsequent release of the Daemons would force Aergoth back to Fort Praius, and besiege the native settlements of the isles. Gadril would also smuggle some of the Daemons to Idryllia, guarding the temple of his Cult of the Unspoken, and resulting in their release into the Underworld to found Karthax.
The first Daemon War would be one of the motivators behind the Companions efforts to create The Shard, and for Idryllian purposes, ceased when The Shard was cast and a barrier placed between Idryllia and the Western Isles. Despite this, the Western Isles own people would continue battling the Daemons throughout the Age of Wars.
Second Daemon War[]
When Stormwind brought down The Shard, the Daemons would soon capitalize. Led by Xylnulifein, having bolstered himself to divinity through worship by hordes of Daemons, some of the Daemon army would leave their siege of Bastion, the last remaining Mariner settlement, to cross the ocean to Idryllia.
Seizing the ruins of Aergoth's port, and their old grounds as the Temple of the Unspoken, Xylnulifein would lead them north through an Underworld passage (now the Iron Dragon Road) to challenge the Idryllian defensive being prepared in Northrealm.
While Xylnulifein clashed with Valdrent, the Dawn Legion, and the other defenders of Northrealm, the initial formation of the New Companions would travel to the Western Isles, following knowledge gleaned from the unsealed Temple Of The Seven. Once there, they would contact Aquar, Elder Dragon of Water, and set a trap in Bastion for the Daemon horde.
The Mariners' would board their fleet into exile, under the newly risen Great Captain, Markus Khamar. Meanwhile the Dawn Knight Athric Tell would lead a minimal defence against the Daemons, luring them into the city. Once entrapped, Aquar sacrificed himself, releasing a tidal wave that submerged Bastion, and also infused with his power, slaying the Daemon hordes by the thousandfold.
On Idryllia, the sudden loss of a huge number of worshippers would depower Xylnulifein immensely, allowing the Dawn Legion to bring him down, and the armies of Northrealm to rout the Daemon forces, ending the invasion.