If your DM doesn't have any objections you could try something like this:
A long time ago, when the world was young, there once was a mighty and evil wizard. He wandered far, but settled down and built his tower in the same lands as two rival tribes, the Dwarves and the Minotaurs. Both groups were clever enough to know that a war between them both would destroy them both, but as they were in fierce competition over the markets for their crafted goods, they didn't co-operate either.
The wizard lusted after the craftsmanship of the Dwarves and so he commanded them to craft him the mightiest blade they had ever made so he could enchant it and so control the world. The Dwarves were no cowards, and made a pact amongst themselves that they would deny the madman till his dying day. Such was this wizard's power, however, that he was able to destroy the entire mountain that the Dwarves lived in, and all of of the Dwarves within it save one. A lone female Minotaur who had seen the destruction saved the Dwarf chieftain's only son, pulling him from the rubble.
The wizard fought long and hard to gain the obedience of the last Dwarf, but when he realised that he would never aid him, he turned to the Minotaurs. They were willing to craft him a mighty and wondrous weapon, but their best work they hid from him. When he found out he flew into a towering rage. However, when he considered how he could repay his enemies, he decided that to be especially cruel, he would have to be subtle. After some evil plans were plotted, he decided he would curse each of his enemies to look like each other. However, he decided to spend a year to see how well it worked on his first subject.
He found the Dwarf chieftain's son and polymorphed him into a Minotaur. The Dwarf realised there was only one person he could trust. He went to the minotaur who had helped him earlier, and bade her free him from his state. After many failed attempts, however, the Dwarf realised the true depth of the wizard's revenge, for he knew that he would never be restored.
All he had left to remember his heritage by was his old Mordenkrad, which was now much to small for him to wield effectively. However, he still had his Dwarven heritage in the form of his smithing, and with what he learned from the Minotaur community he eventually learned how he could make a new weapon of even greater power than before. And thus was the legendary Mordenkrad of the Minotaurs born.
As the Minotaur who Weeps, as he came to be known, made his last and greatest weapon, he poured every drop of his soul into it. When he was finished, his lifeless body keeled over, but his will remained, encased in the steel of his last task. With this great weapon, the Minotaurs were able to destroy the evil wizard as he prepared to mass polymorph them all into Dwarves, as he had planned all along.
Nowadays, no-one in the tribe knows of the history of the Mordenkrad. However, many of them have an unusual proficiency for mining and smithing - the last of the heritage of those who once ruled the mountain.
Basically, in regards to your weapon's backstory: make it epic, make it an adventure in itself, and make it more than a 'Weapon +2' to your character.