The Doomed One.
The Doomed One, also known as the Defiant, Valiance, Knight Gallant and many other names is either very old, or very very young god depending on how you look at it. The god of defiance, last stands and doomed chivalry The Doomed One is powerful but inherently incapable of seeking survival or standing aside if it feels its course is just. This predictably results in its rapid death, however every time it has fallen in time another has risen.
The exact mechanism is unknown - indeed, none truly know if it is the same god reborn, or simply a powerful unclaimed domain recoalescing into another primal god, or something stranger still. What is known is that even when means of dealing true death are brought against the god it seems to have no effect on the cycle, even devouring the god entirely seems to do little to slow down the rise of its successor. Many believe that taking the domain would cease the process, but no creature has dared take a domain so utterly lethal to its holder. Thus The Doomed One, or its line, has continued unbroken for millennia.
Each iteration of the Doomed One appears slightly different, roughly humanoid and clad in some form of bulky, form concealing battered armor and tattered cloak or tabbard. Its panoply is otherwise fluid, always possess some form of weapon and some way to rally or inspire troops. Its armaments has had been everything from axes, to grenade launchers, to entire arsenals strapped to its armored form. The methods by with it inspires has been similarly varied, from horns, to banners, to auto-fanfares or strange icons taken from dead cultures. It has been noted that The Doomed One's power seems to be growing, whether this is some form of inheritance passed down or merely a reflection of the state of the galaxy is unknown.
Typically the Doomed One will last at most a handful of years before falling, throwing itself heedlessly into glorious lost causes. Its most frequent foe by far is the Five, though if this is a result of their many schemes and atrocities or a specific animosity is unknown. When The Doomed One meets its end it stands in the very heart of its domain and its power swells tremendously, the very fate twisting to ensure that its end is a glorious one. Rare indeed are the times it has fallen without extracting at least a symbolic cost from its foe. Be it a minor victory, such as forcing Skulltaker to shatter its skull to best it, or a major one, such as rendering the Dark Singer forever mute. This give The Doomed One a reputation so infamous that often its mere presence can drive back mightiest demons, for none wish to find out what toll it would take from them.
A common beneficiary of its doomed gallantry is other minor gods. As such many other deities will treat it with respect, though more than one minor god has fallen at its hands. It rarely acts in the Materium, but has been known to manifest an avatar at rare times during last stands. Its manifestations sees the defenders rapidly gain a number of surprisingly potent blessings. Fearlessness, immunity to pain, great strength at arms or skill, any number of blessings intended to enable them to sell themselves dearly. However these blessings come at a cost for those who accept them, to take even the lightest of them is a death sentence, as the blessings burn out the minds and souls of those who accept them. Though the Doomed One is too self destructive to truly maintain cults, it is said by some that the Lamenters know much of its lore, and some will call upon it in their final hours. While this may merely be one of many dark rumors that swirl around the cursed chapter, in recent centuries the Doomed One often bears a black and white checkered pattern somewhere on its armor, and it has been known to fight alongside the Lamenters on a number of occasions. It is said The Doomed One manifested a half dozen times on Cadia before it finally fell, forcing Abaddon to expend a great deal of effort to ward the god away from the world, though if this is true few can say. But it is known the Warmaster has promised a great boon to any who can truly end The Doomed One, a prize none of yet to claim.
Presently The Doomed One is dead. During the raid on Nurgle's realm it alone stood against the Plague God's armies when they marched in force to reclaim Plaguefather's holdings, fate twisting so that the only path through which they could reach the fleeing minor gods had a choke point upon which it could stand. Six times did Nurgle's forces crash upon it like a wave of filth, and six times did they retreat terrified and broken. Until at last the Honored Harr'Groth the Rotting Tide strode forth in its countless forms, confident in its victory and a possibility of Exaltation should it succeed. A prize it would never claim, for just as the Doomed One was overwhelmed and its shield shattered, with its last breath it struck with a spear blazing with all the power it had left at the hidden soul of the Rotting Tide, burning it fully from existence. By the time the smoke had cleared, and Nurgle's forces had worked up the nerve to verify the god was dead within its armor, the small gods had long since fled.
If The Doomed One was acting in concert, or merely drawn by a chance to foil the Plague God is not known, or indeed if it is even capable of reason outside of battle. However should the other minor deities find a way to direct The Doomed Ones final stands they would gain a potent shield against future retribution from the Ruinous Powers, though if this is even possible remains to be seen. The danger of courting such an uncompromising entity is not to be discounted either for several of the more loathsome or ruthless minor gods have felt its wrath.