Gloriana-class Battleship
The Gloriana is a rare class of Battleship fielded during the Great Crusade. Created to be the flagships of the Emperor's sons and their Space Marine legions, each and every one is armed to the teeth and heavily armored, on top of being customized depending on each Primarch's needs.
It's important to overstate how massive each individual Gloriana was. They were capable of holding entire expeditionary forces and their support staff and equipment complement. The entirety of a full-strength "modern-day" Chapter of Space Marines would have trouble filling out even a tenth of their cavernous holds.
Not only that, but their firepower was immense as well, with enough lance turrets, macro-cannons, torpedo tubes, and fighter launch bays to fight off entire fleets, with a good chance of the Gloriana coming out on top. Indeed historically, the only way to take out a Gloriana was either through sheer numbers and/or weight of fire, or with another Gloriana.
Chariots of the Primarchs
During the Great Crusade and Horus Heresy, each Legion had a Gloriana class warships as their Legion Flagship, except for the Imperial Fists who had the larger and even more deadly Phalanx.
- Dark Angels: The Invincible Reason
- Emperor's Children: Pride of the Emperor
- Iron Warriors: Iron Blood
- White Scars: Swordstorm
- Space Wolves: Hrafnkel
- Imperial Fists: Eternal Crusader
- Night Lords: Nightfall
- Blood Angels: Red Tear
- Iron Hands: Fist of Iron
- World Eaters: Conqueror (Formerly Adamant Resolve)
- Ultramarines: Macragge's Honour
- Death Guard: Endurance
- Thousand Sons: Photep
- Luna Wolves/Sons of Horus: Vengeful Spirit
- Word Bearers: Fidelitas Lex
- Salamanders: Flamewrought/Fireforge
- Raven Guard: Shadow of the Emperor
- Alpha Legion: Alpha/Beta
Swords of the Emperor
There are few Gloriana-class still in active duty in the modern-day Imperium, and in all cases these honored relics are in command of Astartes Chapters rather than the Imperial Navy. Needless to say, these old survivors have near-legendary reputations or honor rolls that would fill the libraries of entire planetary systems.
The Eternal Crusader, formerly of the Imperial Fists Legion, still lives up to its name, serving as the mobile Chapter-monastery of the Black Templars. Up until recently it had been in pursuit of the infamous Warboss Ghazgul, but has since then been deployed for the Cardinal Worlds Crusade in the wake of the formation of the Great Rift.
The Invincible Reason still remains the flagship of the Dark Angels, though in practice it mostly serves as the escort of The Rock in its travels through the galaxy. Much like the Rock, it's currently deployed in Imperium Nihilus.
The Macragge's Honour of the Ultramarines by comparison spent much of the intervening millennia as a spaceborne shrine to their Primarch, and has only relatively recently returned to active service. For a time it was lost to the Red Corsairs in the wake of the disastrous Terran Crusade, but has since then been recovered, and once more serves as the Lord Commander's flagship.
Finally there's the Chronicle of Ashes, formerly of the Word Bearers, it was captured, renamed the Lex Talonis, and had since then served the Nemesis Chapter as their flagship.
Echoes of the Old War
There are several Gloriana-class ships known to be still in service to the dark powers.
The most infamous of them all, the Vengeful Spirit, flagship of the archtraitor Horus Lupercal, still bedevils the Imperium well into the current age. Now under the command of his successor, the Warmaster Abaddon the Despoiler, the ship leads the fleets of the Black Legion in their rampage out of the Eye of Terror after the fall of Cadia.
Likewise the Magna Tyrannis, renamed the Harbinger of Doom, flies the colors of the Black Legion.
Mortarion still commands the plague-ridden Endurance, and used it during the Plague Wars to terrorize the worlds of Ultramar. Likewise the Conqueror still remains under the control of the World Eaters, who use it to wreak havoc in Khorne's name.
It's conceivable that the Alpha and Beta of the Legion that doesn't exist are still in operation, but given how that Legion functions, no one knows for sure.
In Battlefleet Gothic
The Gloriana-class was not statted out when the specialist game was first released. Eventually we were provided the stats of the Vengeful Spirit, but it was underwhelming to say the least, being a slightly more boarding-effective Chaos Battle Barge. It doesn't even get Chaos-specific benefits, especially compared to the other Chaos God-aligned battles barges statted out (see the Terminus Est of the Death Guard).
Not exacly reflecting its reputation as a ship capable of fending off entire Loyalist fleets, and enduring the world-breaking duel between the Emperor and Horus.
The sidebar to the Vengeful Spirit also mentions its equivalents still serving the Imperium are considered "relic" Battle Barges that can customize their weapon load-out before deployment for free, and come with a Space Marine crew by default. Again, a far cry from what's expected of a ship class that can fend off fleets on its own.
In Battlefleet Gothic: Armada 2
The ship class makes its debut in the sequel game, with Macragge's Honour representing the Imperium while the Vengeful Spirit appears on the Chaos side. Each is easily twice as powerful and durable as a "regular" battleship of their faction. Just don't send them without escorts however, as they can still be overwhelmed by multiple ships ganging up on them.