"The Sons of Dorn are too stubborn to die. Why should his regal daughter prove different?"
- – Captain Tor Gardon of the Imperial Fists 3rd Company, after using The Phalanx to ram a Blackstone Fortress into oblivion over Cadia.
"That's no moon. It's a space station."
- – Obi-Wan Kenobi
Dorn's Proud Daughter
Much like many things that predate Imperial history, the origin of the Phalanx is shrouded in legend and myth. One source says that Rogal Dorn himself built the battle fortress once he became the over-all ruler of the ice world of Inwit, while another states that it was predates the Primarch's arrival on that world, and was just added as part of his inheritance once he ascended to the position of planetary warlord. What is important is by the time the Emperor and his expedition finally reunited with Dorn, the Phalanx was already under the latter's command.
Since then the Phalanx had served as the center of operations for Dorn and his Legion (and later Chapter), the Imperial Fists. And while the Fists count many worlds as its recruiting grounds, with Holy Terra itself listed as its "official" home world, the Phalanx would always be its home.
The Gathering Storm
Close to the end of the 41st millennium, the fortress would again be subjected to another daemonic invasion. This time around only the 3rd Company led by Brother-Captain Tor Garadon and half of the 1st Company's Terminators were available to defend it. Leading the incursion were Darnath Lysander's old foe, the Iron Warriors Warsmith Shon'tu and the Daemon Prince Be'lakor, who sought to outdo Abaddon by conquering the Phalanx and dropping it on the Imperial Palace.
Garadon and his men fought hard, but despite their determination they were eventually pushed back to one final redoubt. Prepared for what looked like a last stand, it instead became a counter-attack when the Legion of the Damned appeared en-masse to push back the daemons. Shon'tu was slain, and Be'lakor banished, but the Phalanx was wounded by the assault, as it was forced to purge ten percent of its own superstructure to prevent the spread of Chaos corruption.
As its fleet master Trevaux urged Garadon to turn the fortress to the Ring of Iron at Mars for repairs, a desperate transmission from Cadia was received. It was then that Garadon knew where his duty lay. After the most basic repairs were made, the Phalanx translated into the Warp, on course for the beleaguered fortress-world.
Dorn's Mailed Fist
The arrival of the Phalanx caught the Chaos fleet above Cadia by complete surprise. Taking advantage of the confusion, the star fort plowed through the traitor formations with the aim of taking out the Blackstone Fortress. It was an almost-suicidal charge, and there was no guarantee that even the Phalanx's main guns could hurt the Blackstone, but Garadon ordered the Phalanx on anyway.
Then the shields on one part of the Will of Eternity went down (thanks to sabotage by Wolf Lord Sven Bloodhowl). Seizing the chance, the Phalanx opened up with all of its batteries. As the Blackstone Fortress died, the Phalanx limped away, and took a position above Cadia's equator to lick its wounds, right about where the Elysion Pylon Fields were. It would later provide support there via orbital bombardment once the Imperial forces moved to defend the Pylons.
Shield of the Diaspora
When the secret boosters triggered to plunge the remains of the Blackstone Fortress into the planet, Garadon had all of the Phalanx's guns open fire in a bid to destroy the continent-sized mass before it was captured in Cadia's gravity well. It didn't work. The piece struck Cadia, hard, and the planet burned.
Against all odds there were still survivors, and as transports full of these rose up desperately through daemon-infested skies, it was to the Phalanx they approached for protection. It waited until the last moment, but when the final transport left the doomed planet, the Phalanx led the ragged flotilla of ships toward the edge of the Cadian system, where they could finally escape into the Warp.
Wounded, But Unbowed
The Phalanx would finally return to the Sol system, and would remain in orbit between Terra and Luna undergoing repairs when the Battle of Luna happened, as the remnants of the Terran Crusade under command of Roboute Guilliman burst out from a hidden Webway gate, with the daemonic forces of Magnus the Red in pursuit. Although it would scramble several Thunderhawk gunships to reinforce the Imperial forces, it would not move from orbit until all the repairs were done.
In the current age, the Phalanx still keeps vigil over the crown world, ordered to defend Terra by the Lord Commander himself.
As a little aside, a contingent of Custodes covertly snuck in during the repairs of the Phalanx, and are currently squatting there like shiny gold-plated gopniks (not that hard), ready to sabotage the fortress if it gets used against Terra.
As of the 8th Edition Imperial Fists supplement, it's been shown that the venerable battle fortress has been restored to close-but-not-quite-full operational capabilities thanks to archeotech donated by the Adeptus Custodes. In exchange however, the Fists have begrudgingly accepted a visible contingent of Custodians on board (on top of the others that the Fists suspect already slipped aboard beforehand).
Its local defense force had also swelled thanks to many survivors of the Cadian Diaspora volunteering to safeguard it, as the feel that they owe the Chapter their lives after what it had done to aid the escape from the Cadian gate. Presumably, like far smaller Imperial ships a good chunk of these are permanent residents.
The Fortress Itself
The Phalanx is the size of a moon/planetoid, dwarfing even the Ramilies Star Forts that the Imperium uses to defend its holdings. At its heyday, its docks could service entire fleets worth of ships, and even in the present day is enough to hold the Imperial Fists ships with space to spare.
Speaking of its docks, various sources note that, due to its huge size, it had developed its own unique ecosystem, with flora and fauna that had evolved to live there.
On top of this prime real estate, the Phalanx boasts a ludicrous amount of weaponry, point defenses, armor, and shielding. Multiple fleets would be hard-pressed to equal the sheer amount of hurt the battle fortress can dish out, and as the Battle of Cadia proved, even when bloodied and battered it can give as good as it got in return, and then some.
Just like any good scion of Dorn would.
That said, it's also been noted that the Phalanx is a shadow of its old self, and many of its areas lie empty and neglected due to it being far too large for the modern Imperial Fists to use in full. Many of the technology that went into its construction are also either no longer understood, or can't be easily replicated or replaced by even the Adeptus Mechanicus. Indeed its likely that the systems damaged before and during the Battle of Cadia are truly irreplacable.
In Battlefleet Gothic
The Phalanx was not statted out for Battlefield Gothic, though given how a Gloriana-class Battleship (the Vengeful Spirit) was statted out simply as the equivalent of a Battle Barge, it's not hard to imagine the Phalanx given the stats perhaps of a souped-up Ramilies Starfort.
Realistically any kind of game stats for the Phalanx have to be equal at least to the "Activated" Blackstone Fortress.
In Battlefleet Gothic: Armada 2
The Phalanx makes its appearance in the tutorial section of the second game. Much like its role during the Gathering Storm, the battle fortress makes its surprise appearance in Cadia's orbit, throwing a wrench in the Warmaster's plans as it proceeds to plow its way through the Black Fleet and actually destroy the Blackstone Fortress Will of Eternity. All this while under your control.
Unlike how it was described in The Fall of Cadia book, the Phalanx in-game is effectively untouchable by anything it faces during its run toward the Blackstone Fortress, with 7200 HP, three times the shields of a battleship, and the default 83% reduction on all sides that a Space Marine unit has by default. Even the Will of Eternitys iconic Warp Beam, which we have to remind everyone was made to kill C'tan, barely scratches its shields.
In the Necron campaign, Trazyn the Infinite reveals that the Phalanx houses the trionic activator that Phaeron Kephrekh requires to activate the Dark Throne. Nemesor Amarkun is sent to retrieve it with Trazyn as his "guest" and successfully sends warriors to board the Phalanx, whose shields are occasionally disrupted by solar eruptions. Unable to evacuate the warriors due to the "agitated" humans, Trazyn boards the Phalanx and unleashes a tesseract labyrinth containing a Tyranid swarm of "quite humorous extent", allowing the device to be extracted and resulting in the complete destruction of the Phalanx.
It also proceeds to fuck with you in the Chaos Campaign, by every so often picking an Imperial-owned System and causing the Fleets there to spew attacks at you, as well as allowing Fleets within to reinforce and be created every turn. Stopping it from doing this requires that you dive in, deal with it, and whatever fleets and platforms are defending it. Even then, it cannot be brought below 1 Hit Point and boarding parties cannot prevent it's escape.