Devastation of Baal

The Devastation of Baal (First iteration)
Date	~999.M41
Scale	Planetary
Theatre	Third Tyrannic War
Status	Tyranid victory*
Belligerents
Tyranids	The Imperium of Man
Commanders and Leaders
The Swarmlord	Dante, Gabriel Seth, Ercon, Scaraban, Erden Cleeve, Follordark, Sentor Jool, Kaan, Raxiatel, Zedrenael, Zargo, Sendroth, Hajjin, Quaeston, and later Roboute Guilliman
Strength
Hive Fleet Leviathan	Blood Angels, Flesh Tearers, Knights of Blood, Blood Drinkers, Carmine Blades, Angels Encarmine, Angels Excelsis, Blood Swords, Sable Brotherhood, Angels of Light, Flesh Eaters, Charnel Guard, Golden Sons, Angels Penitent, Sanguine Host, Angels Glorious, Burning Blood, Brothers of Jarad, Blood Wings, and Angels Vermillion Space Marine Chapters, Legion of the Damned, and later forces from the Indomitus Crusade
Losses
Very high	At least five Chapter Masters killed, Angels Excelsis, Golden Sons and 6 other Chapters destroyed, 6 more Chapters almost wiped out, remaining Chapters reduced to less than half of their number, all but 1 of the Sanguinary Guard
Outcome
*Battle inconclusive; interrupted by the formation of the Great Rift

The Devastation of Baal was a gigantic three-way free-for-all battle between the Imperium of Man, Tyranids, and (later) a whole fuckton of Daemons for control of Baal and its two moons. It was one of the most devastating events of the Third Tyrannic War for all parties involved, and was directly affected by the galaxy-changing Noctis Aeterna.

Prelude

The Cryptus Campaign

In 998.M41, the Blood Angels led a spirited defense of the Cryptus System during a grueling campaign against the onslaught of Hive Fleet Leviathan. With the reputable defenses of the Cryptan Shield having been severely fucked by the Hive Fleet, Chapter Master Dante quickly gathered an alliance of his own chapter, the Flesh Tearers Chapter, dozens of Imperial Guard regiments, the Order of the Sacred Rose, an entire Imperial Navy Battlefleet, and even a force of Tempestus Scions. Despite this massive Imperial presence, the Cryptus System fell anyways in just two days. In the ensuing Imperial counterattack, Dante forged an alliance with none other than the Necrons of Anrakyr the Traveller, who happened to have been in the system awakening legions of the Mephrit Dynasty. Who the hell let Ward get his hands on this narrative!? Together, the Blood Angels and the Necrons managed to activate an ancient Necrontyr doomsday weapon called the Magnovitrium, which ignited the core of the gas giant Aeros to send a massive pillar of flame across the system, incinerating the Tyranid fleet as well as some unfortunate Imperial soldiers, citizens, Sisters of Battle, and Space Marines (of course, the Necrons phased out right before the weapon activated). With Hive Fleet Leviathan having been dealt a huge blow, as well as all of the biomass in the Cryptus System having been burnt to a cinder, the Imperial forces (or what was left of them) withdrew. However, the Cryptus Campaign only delayed Leviathan's inevitable advance towards Baal.

Imperial Preparations

Realizing the incoming threat, Dante forged yet another alliance of Imperial forces. This time, he gathered the support of all 20 Blood Angels Successor Chapters, barring the Lamenters (you can't hold it against those poor bastards; by this point they're severely mauled, and they couldn't reach Baal in time even if they were still surviving to begin with). Even the renegade Knights of Blood Chapter answered the call for aid, but they wisely decided to fight separately from their brethren. Dante launched pre-emptive attacks against Leviathan, and managed to inflict losses against hundreds of splinter fleets. When the main Hive Fleet reached Baal, a mixed fleet of the assembled Chapters' Battle Barges and cruisers launched salvos of nuclear-tipped warheads directly into the mass of organic ships. But eventually, all of the Space Marines' vessels were overwhelmed by the sheer amount of Hive Ships filling the space around Baal and its moons, and the Imperial forces on the ground could only watch as millions of living missiles rained down on them past the wreckage of once-mighty Imperial ships.

The Battle

The Tyranids sent nineteen waves of merciless attacks, with each one being even larger than the last. These waves were driven off at great cost by the Imperial forces, with five Chapter Masters falling in these battles; three of the Chapter Masters were killed in the bloody Battle at the Dome of Angels alone. The Space Marines initially held their own thanks to recommissioning all of their powerful Heresy-era fortifications and weaponry, but the tide inevitably began to turn when the Swarmlord itself began leading the Tyranid hordes. Despite sustaining horrific wounds, Dante managed to slay the Swarmlord in an epic duel. Eventually though, as Baal itself was overwhelmed, the Tyranids began the process of draining the world and her moons of biomass, absorbing even the rad-poisoned desert of Baal Secundus. With the Imperial defenses falling little by little, the battered defenders retreated to the Blood Angels' Fortress-Monastery, with the planet close to destruction. But then...

The Devastation of Baal (Second iteration)
Date	~999.M41
Scale	Planetary
Theatre	Third Tyrannic War
Status	Imperial Pyrrhic victory*
Belligerents
Chaos	Tyranids
Commanders and Leaders
Ka'Bandha	The Swarmlord
Strength
Hordes of Khornate Daemons	Hive Fleet Leviathan
Losses
All killed or retreated back into the Warp	Near-total annihilation
Outcome
*Imperium claims victory with the mutual destruction of the Tyranids and Daemons; Baal and its moons are almost completely devastated