Hive Fleet Naga was a Tyranid Hive Fleet, relatively small in comparison to other Hive Fleets, that invaded the Galaxy from the Eastern Fringe somewhere to the galactic-north of the Tau Empire in 801.M41, destroying countless alien domains and devouring several Eldar worlds. The Eldar refereed to Naga as Shai'naid, the Endlessly Winding Serpent, and fleets from the Malan'tai, Iyanden, and Idharae Craftworlds eventually eliminated Naga in 812.M41. But not before the Tyranid Hive Mind absorbed several dozen samples of new biomass, including the DNA of the Eldar and also gained valuable experience of Eldar military capabilities.
The Ulumeathic League
Naga's first major attack on the galaxy was in 012801.M41 when it fell upon the Ulumeathic League and within days destroyed its four outer colonies. Genestealers managed to infiltrate the few refugee freighters that escaped the colonies, and after two days of the freighters arriving at the Varreyan space docks, all contact with the complex was lost as the Genestealers took their toll. A fleet from the Varreyan planet was sent to destroy the entire facility from orbit. However doing so crippled their own ability to rearm and refit any further spacecraft in the region. Naga for the next five years from 802.M41 to 807.M41 committed its forces to consuming several minor alien races on the edge of Ulumeathi space. Then in 692808.M41 Naga returned to assault the Ulumeathic League again, absorbing the League's Throneworld before the remaining colonies declared a full quarantine.
Naga attacked areas of space with little Imperial presence and so at first, rather than admitting a new Tyranid threat was entering the galaxy, the majority of the Administratum believed it to be a splintered tendril of Hive Fleet Behemoth that would soon die off from starvation. It wasn't until two hundred years later when Hive Fleet Kraken emerged that it was understood there were other Tyranid Hive Fleets poised to enter the galaxy.
The only dense concentration of Imperial worlds that was invaded was the Ybaric Cluster. Located beyond the Eastern Fringe of the Vidar Sector, they had been isolated by Warp Storms for several centuries. When the warp storms ceased, the worlds were found to be devoid of life and Hive Fleet Naga was gone.
As Naga assaulted the Ulumeathic League, rumours reached the Ybaric Cluster of an approaching alien menace and a cult known as the Cult of Veiled Oblivion grew in membership all across the cluster. In 909801.M41 Naga invaded the world of Crimsonfall. The nearby world of Tern sent reinforcements including a company of neglected but operational Cadian Baneblades. The Imperial defenders rallied behind these powerful tanks, but they were quickly reduced to scrap by a brood of Tyrannofexes. Further reinforcements were shipped from Tern only to be ambushed enroute by Dark Eldar raiders, who in turn were caught and destroyed in Naga's advance. The coremost world of Silax, which had fallen into anarchy as the Cult of Veiled Oblivion gained prominence, refused to send aid to Crimsonfall. In 701802.M41 Naga devoured Crimsonfall and then continued on to Tern which, having sent many of its forces to Crimsonfall, was easily overcome. In 001808.M41 Naga arrived over Silax. On the planet's surface members of its population, including the planetary governor, revealed their true allegiance to the Cult of Veiled Oblivion and took arms against the loyal defence forces. Both sides were devoured when Naga descended on the planet.
The elimination of Hive Fleet Naga had little to do with the Imperium and was chiefly at the hands of the Eldar. Naga's first attack on the Eldar was in 640801.M41 when it suddenly devoured the Exodite World of Tar-Etenil. The death throes of the planet projected a psychic shockwave that crippled the World Spirits of several neighboring Exodite worlds, leaving them easy prey for Naga.
In 131809.M41 Eldar fleets from Malan'tai and Idharae had gathered to attack Naga's leading elements. Naga responded by splintering its forces into two tendrils, the first continuing ahead to the Exodite world of Halathel, and the second turning back rimwards towards the Maiden World of Eth-aelas.
The Exodites of Halathel were unable to abandon their kin's souls contained in the planet's Worldspirit and so began to fortify the Worldspirit shrine while begging with nearby Craftworlds for assistance. The first tendril outsripped the crippled Malan'tai fleet to arrive at Halathel first, and in 801809.M41 launched waves of Mycetic Spores onto the planet. The defenders withdraw to the Worldspirit shrine to make a desperate last stand. Thousands of Termagants and Hormagaunts hurled themselves at the shrine's walls to deplete the enemy's ammunition stores, but the Exodites held out for three days and three nights until the Tyranids ceased their assault. When the Tyrand's launched their second siege of the shrine, it absolutely dwarfed the first in numbers and fury. By dusk, Naga had broken through the outer defences and began pouring through the breaches. Exodite Lord Wei-yannil stood against the Hive Tyrant leading the assault to buy time for the Eldar to withdraw. Both combatants were slain in the fight - Wei-yannil bringing down the Hive Tyrant, but then was torn apart by the beast's rampaging Tyrant Guard. With the Synapse creatures death the Tyranid swarm lost cohesion and withdrew.
Above Halathel, a fresh fleet from Iyanden commanded by Admiral Draech engaged the Hive Ships orbiting the planet. Draech's flagship, the Auspicious Illumination of Eternity was destroyed early in the battle. Command of the fleet was quickly seized by his second in command, a young Prince Yriel, who successfully lead the Iyanden fleet to victory. On the planet, the Worldspirit shrine was finally taken when dozens of Trygons emerged from burrowed tunnels beneath the shrine's walls. In less than an hour, the Eldar defenders were massacred and the Worldspirit was destroyed. Yriel, overwhelmed by rage, gave the order to scour all life from Halathel. The survivors of the Malan'tai fleet arrived to join in the cleansing.
Naga's second tendril was caught by the Idharae fleet on the outer edges of the Eth-aelas system and, finding itself cornered and outgunned, was destroyed. But not before it managed to send several thousand Mycetic Spores raining down on Eth-aelas. Idharae ground forces descend to the planet, determined not to lose another of their worlds, and set about the long task of freeing the world from the Tyranids. Having dealt with the first tendril, Yriel's forces arrived and joined in. After the last Hive Tyrant was finally slain at the battle of Sorrowforge Pinnacle, the remaining Tyranids were left leaderless and easy targets.
Naga's presence as a unified force was ended. In all the Eldar lost two Maiden worlds and seven Exodite worlds.
However before Naga's full destruction, a lone and wounded bio-ship stumbled across the Malan'tai Craftworld and in its death throes launched its last Mycetic Spores. They went undetected until they hit the Craftworld itself. The Eldar struggle to fight back the invading creatures, but a unique vampiric Zoanthrope wipes the entire Matan'tai Craftworld of all life, taking the name the Doom of Malan'tai.