As recounted in The Histories of the Unconquerable Realm, the first Duke, Beren, fought valiantly against foul ambushers within the Forest of Arden and was the lone survivor of his party, sorely injured and lost. A vision of a white hart led him to safety, and it is this occurrence that is remembered and honoured in the heraldry of Beren's descendants.
Duke Hagen is a Grail Knight and close companion of King Louen. The Duke is utterly commited to the ideals of knighthood, even more so than most Grail Knights, and the King knows that he can rely on hagen for advice on the chivalrous thing to do. Duke hagen's decisions are unclouded by concern for consequences, so the King does not always take his advice; King Louen is mindful that it is sometimes better to act unchivalrously than to throw away hundreds of lives.
The Duke sometimes has troubles with these decisions. He is aware that the King has wider responsibilities and still trusts his friend's commitment to the ideals of chivalry and honour. Still, every time Louen compromises those principles for the greater good, the Duke worries about the chances of corruption.
Hagen hardly ever leaves Louen's side. The King likes to have the Duke's advice readily available, and the Duke, though he would scarcely admit it even to himself, wants to keep an eye on the King.
The Horrors of the Arden Forest
Many tales are told of when the sun was swallowed by blackest darkness and all of Bretonnia was enshrouded beneath unnatural gloom.
What became of the battle that darkened Bretonnia began when Duke Hagen and his knights ousted Lord Malvous's coven of Vampires from the cursed lands of Mousillon. Vowing to rid the land of his evil, Hagen pursued Malvous and his vampires across the kingdom, seeking to end his reign of blood sucking horror. The trail was not hard to follow, rumours of unholy Black Coach apparitions rumbling down the road, sightings of the walking dead and a string of small villages being entirely emptied. One by one the Vampires were hunted down until only Malvous remained. The Vampire Lord fled into the Forest of Arden, hoping to escape the Duke's holy wrath.
The foul Vampire discovered an ancient barrow deep within the forest, through force of will the vampire stripped away the tombs ancient wards and penetrated deep into the oldest of the barrows. In doing so a well of dark energy, which the ancient runes had siphoned and stored for ages, was released. Immediately a power surge tore through the winds of magic and set into place a series of devastating events. Sending dark smoke across the land and shrouding the forest in a blanket of darkness. 
In the woods nearby Hagen rode at the forefront of his knights, guided on the trail by a Fay Enchantress. At long last Hagen spied his quarry, for the hated vampire was looming over the trees atop a pillar of skulls. A halo of dark energy surrounded the foul creature as he chanted a hellish spell. Deep blasts of warhorns and the distant roars of savage beasts signalled monstrous things stirring in the darkened woods. The Knights behind the Duke, their bright heraldry vivid even in the supernatural gloom, fought to steady there fearful horses. Duke Hagen prayed for the Lady's protection and his loud voice carried over the howling winds and steadied both his knights and their mounts. The Bretonnian army advanced , coming to clearing at the foot of the cairns in time to see the hills above them swarming with an army of ancient undead. 
It was a nightmarish scene as thousands of Skeletons rose from the ground and formed up beneath fell banners. Wight Kings led each regiment, their ornate crowns and ancient armour glowing eerily in the darkness. Purplish clouds swirled ominously low over the field of battle, forming around the vile Lord Malvous. Across Bretonnia all rays of sunlight disappeared into a thickening gloom. Iradescent bolts forked down from the sky, momentarily lighting the battlefield. Where the magical lightning struck, the trees started to move and come alive, uprooting themselves to join the fight, lumbering towards the Knights. Undaunted, Duke Hagen and his knights lowered their lances and crashed into cadaverous creatures and animated wood spirits alike. Bones and splintered tree limbs flew as the pride of Bretonnia clove through the foe.
As the knights charged forwards, a new and more terrible threat emerged. Roused by the storm, the monsters of the forest appeared from their noisome lairs. Infused with the power of the dark tempest, Lord Malvous found it child's play to ensnare the great predator's minds, binding them to his own will. The first of these creatures to emerge was a pride of three Manticores. Normally solitary creatures, a single Manticore can spell doom for an entire settlement, but three together was enough to make the boldest heart quail. Bounding forward and gliding on leathery wings, the enraged beasts closed with horrifying speed, hitting the knights like a thunderclap. Great claws tore armour and unseated knights with ease, while the terror filled whinnies of horses could be heard above the sound of crunching bones beneath powerful jaws. The swirling melee quickly developed into the fiercest battle Duke Hagen had ever witnessed, yet the Bretonnians held on.
As Hagen countered one threat, another emerged - rising out the cairns or pushing from the forest. The very land seemed infused with evil spirits as the tree roots writhed, snaking out to strangle the wounded. The woods were filled with the bellows of monsters and the cries of the injured and always the deep, rumbling thunder of the unnatural storm. Flocks of Harpies screeched down from the treetops to tear proud knights from atop their saddles and carry them off to the black skies. The unmistakable sound of armoured forms being dropped from on high and hitting the forest floor with a sickening clang, echoed throughout the forest. A towering Giant, a behemoth beyond any Duke Hagen had seen before, smashed aside trees to wade into the ensuing battle. The creature was so tall that its upper half was lost in the gloom, and only when he bent down to scoop victims into his mouth could his true vastness be seen. Into this maelstrom of combat the foul Vampire summoned black lightning to rip horse and man apart, while spectral warriors swept from the unnatural dark to harry the beleaguered Bretonnians. Black cloaked apparitions that resembled death itself stalked around the battlefield, mowing down knights with every stroke of their sweeping scythes. With grim realisation, Duke Hagen knew that he could not win this battle, yet his harsh realisation bred not despair, but a resolute desire to go down fighting, to take such a toll upon the enemy as to be remembered in legend and song.
Without warning the Wood Elves of Athel Loren appeared all over the battlefield, as if summoned by magic. Green clad archers sent hails of arrows to slay two of the Manticores, and the Wood Elf cavalry began to charge the undead. A massive Forest Dragon appeared and landed with a rash amongst the skeleton horde, shearing bones with every strike. For a moment the Duke felt hope renewed, but that was soon lost when the foliage parted to reveal a massive three headed monster from the fables of his forefathers: A Chimera had come, and once again the fortunes of battle shifted. The lumbering beast destroyed every elf in its path, its serpentine head lashing out so its dragon head could snap a knight in twain, while its other head skewered yet more elves upon its mighty horns, and the lion head roared its triumph over the broken remains.
Even thus, in the darkest of hours beneath unwholesome skies, all was not yet lost. Duke Hagen looked on as an elven sorceress teleported the Vampire Lord from atop his pillar of skulls, straight into the centre of the battlefield. Duke Hagen goaded his warhorse into a charge, his blessed lance hit Lord Malvous straight in the chest, smashing aside ribs and skewering the vampire in its non beating heart. With an unholy howl, ashen skin shrivelled before Hagen's eyes and bones turned to dust, leaving behind only a mouldering mound topped by a fanged skull.
Before the last echo of Malvous's death shriek had finished ringing through the forest, the undead army collapsed like marionettes shorn of their strings. A trio of elven sorceresses gathered at the broken cairns, singing songs in a language Hagen could not understand. As they sang the unnatural clouds parted, and light once more flooded into Bretonnia. The monsters fled back into the forest followed by the arrows of the elves, Hagen dismounted his steed to examine the remains of the cursed vampire he had slain. He turned to see the elven spellcasters staring at him, and that was the last he remembered before waking up within the bright morning light, on a hill outside the forest. In his gauntleted hand he held the skull of the Vampire Lord...