The Sunstar Flamestrike is the heirloom of the Emperor and his septre, as well as his weapon of choice. The Sunstar Flamestrike is the symbol of the authority of the Emperor and is only permissible to be wielded by the true inheritor of the imperial throne.
The Sunstar Flamestrike came from two weapons, the first, most reknown was the Felo'melorn, or Flamestrike, the sword of the Dath'Remar Sunstrider, the first High Elf and heirloom of the Sunstrider Dynasty. The other, was the septre of the vile death knight, Teron Gorefiend.
The main structure of the mace-septre, came from the Gorefiend, the weapon of the Death Knight, Teron Gorefiend. After the defeat of Illidan at the Black Temple, Gorefiend himself was also slain but mace was recovered by the Scryers and taken back to Shattrath. It was infused with enormous demonic powers which was one of the reasons for the Death Knight's unnaturally powerful abilities. The weapon was never given a true name, but was named after its previous owner.
Seeking to cleanse the corruption, the Scryers took the septre to the Naaru. They exorcised the demonic corruption, revealing a weapon of light and arcane purity. They presented it, as a gift to the Emperor.
After the death of Kael'thas Sunstrider, the sword was recovered by the Emperor and taken as his personal weapon. However, to signify a renewal of the Empire, the Emperor decided to have the blade reforged and strengthened. Combining the Gorefiend with the Flamestrike the two weapons were merged into a single septre-mace that would be created. The two weapons were literally shattered then remade from their core elements. The intensive heat and power required to remake the weapon could only be done in the Sunwell.
The Elven Swordsmiths and Arcanists took the two weapons into the Sunwell and over the course of weeks, melted and tempered the metals, and with each beating of the hammer on the weapon it became stronger and stronger. Like all elven runeblades, the metals only become more powerful with every reforging. The Emperor was presented with the Sunstar Flamestrike, and it was enchanted to protect its wielder with a powerful magical shield.