Macragge's Honour is a Gloriana-class Battleship. It served as the flagship of the Ultramarines legion during the Great Crusade and the Horus Heresy. As of M42, it once more serves as Lord Commander Roboute Guilliman's flagship.
During the Horus Heresy
Under the command of Guilliman, Chapter Master Marius Gage, and then-sergeant Aeonid Thiel, its daemonic invaders were driven back. Guilliman then led a force of fifty terminators in a teleport attack to confront Kor Phaeron and his Gal Vorbak on the orbital platform they occupied.
The Word Bearers were eventually forced to flee on their flagship the Infidus Imperator. Guilliman ordered Gage to pursue the traitor flagship, which led to a running battle between the two battleships. At great cost the Imperator was destroyed (which among things, involved the sensor-blinded Honour homing in on the teleport beacon of the sole remaining Ultramarine Terminator still alive that was still fighting of the scores that initially boarded the Imperator to aim its lance batteries), but the Honour was left in the warp without an accurate way of getting home, meaning none of the surviving crew had any idea if and when they would return to Macragge.
It somehow survived its ordeal in the warp, but by the time it managed to limp back into the Ultramar system, the Horus Heresy was long over, and the battles of the Scouring already fading from memory.
The ship was eventually repaired, but it never served in an active capacity in the Ultramarines fleet. In the ten millenia since then, it had become a shrine of sorts, a sad relic of a much brighter time.
Upon Guilliman's return in the aftermath of the 13th Black Crusade, the Honour would be re-purposed once more as the Primarch's flagship. From there he would lead the campaign to first clear the Ultramar system of invaders, and later on, the burgeoning fleet that would make up the Terran Crusade.
On the way to Holy Terra, the Crusade got waylaid in the depths of the Maelstrom by the forces of the Red Corsairs, and the Honour had to be surrendered to the renegades when the Primarch was captured by Kairos Fateweaver. As Guilliman and the remnants of the Crusade followed Cypher and the Harlequins through the Webway, the Honour was one of the many ships that had to be left behind.
It is unknown just how long it stayed under Huron Blackheart's possession, but by the time of the Indomitus Crusade the Honour was once again under Guilliman's command. One can only assume that the Lord Commander suggested to a certain Chapter that it should be "graciously gifted" back to the Ultramarines.
Since then it has continued to serve as Guilliman's flagship. It's been heavily retrofited and upgraded using tech care of Belisarius Cawl. Guilliman personally reckons that its performance increased about 10% since the modifications were done.
In Print Media
The Macragge's Honour features heavily in the novels Know No Fear and Dark Imperium, and the graphic novel Macragge's Honour. It is examined more closely in the new Guy Haley novel Dark Imperium: Plague Wars.
In Battlefleet Gothic: Armada 2
The Macragge's Honour appears as a usable Titan-class unit for the Imperium in Battlefleet Gothic Armada II, where it arrives to reinforce Admiral Spire's fleet against the Iron Warriors and the Planet Killer.
The Honour itself has twice the Hull Points and Shields as a regular Battleship at a staggering 743 points, as well as the Armor values of Space Marine ships on all sides. It is armed by long-ranged Plasma Macrocannon broadsides, long-ranged dorsal mounted Lance batteries, and a total of six Launch Bays to sortie Thunderhawks or Stormtalons. In engagements at full strength and the odd escort, it's pretty much it's own fleet. You are free to reinforce it with some other ships from the Ultrasmurfs but hey, at least we have a Primarch fighting with Spire. Hell, you can pretty much use it to ram Failadon into oblivion if you wanted.
It benefits from 8 Torpedo Tubes; they support the standard high explosive payload and Astartes Boarding Torpedoes, though not Melta payloads.
The Chaos campaign sees the Honour (and Guilliman) fall victim to the Word Bearer's wacky escapades, as they clear the ship of all lifeforms and bring Bobby G to Abaddon in chains. The Macragge's Honour is given a new lick of paint and a picket fence (and a small dose of daemonic infestation) and is relaunched under the name Monarchia Redeemed.