Footage from SE
The Sword of the Septims
Whether the sword dates back to Tiber Septim himself or not is ambiguous from all records, with the first obscure inventory reference coming around 220 years after his Ascension.
Regardless, the blade was undoubtedly as powerful when wielded as it was symbolically, having been forged, enhanced, and enchanted by the best that generations of Emperors could afford. It was said to have been tempered with the Dragonfires of the Temple of The One, empowering both it's edge and flame.
Last reported, Third Era, 433
Geldall, Uriel VII's eldest was assassinated with the sword in his possession; he had begun to take on several of the symbols of the Emperor at this time.
According to Grandmaster Jauffre's journal, he instructed one of his most trusted agents, Epespadius, to take the blade to Skyrim. He had realized after the loss of the Emperor, his heirs, and the Amulet of Kings, that the Blades could not provide sufficient protection, concealment would be the better option. The sword was to be hidden until the time came to summon the agent back, and crown the new heir. 
When the Grandmaster sent his summons however, as Martin Septim was preparing to declare himself at the Temple of The One, the agent bearing the blade was silent.  The messenger sent to his last known location, 
a pass southwest of the Bloodlet Throne Ruin, had vanished as well.
In the disorder following the end of the Septim dynasty, even as the Blades coalesced around the Medes, it seems recovering this lost artifact fell by the wayside.
To find, read the old note on Esbern’s desk where he can be first found in Riften, or his desk by Alduin's wall in Sky Haven Temple (a copy of the note that starts the short quest is present in both locations).
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There are a few concurrent powerful enchantments as this sword of Emperors. The blade can be ignited with flames when activated with a lesser power that is auto-equipped when the sword is equipped. This deals fire damage (20 over 2 seconds), illuminates, and creates a fire damage explosion on power attacks (45– normal enchantment of 20 damage does not fire simultaneously). Performing a power bash will release a light stagger spell. Blocking casts an Ebony flesh spell upon the wielder as long as the block is held, reducing the damage of both physical and magical attacks to provide the wielder of this two-handed blade similar protection to that afforded by a shield when blocking (Can be converted to a 1-handed at a forge if desired but this was envisioned as a two-hander with the gameplay and enchantments).  The base damage is equal to an ebony greatsword and it is fully temperable with the ebony smithing perk. Overall this is a powerful but not OP weapon intended for higher level characters.
I desired a two-handed weapon that would give enchanted protection allowing the wielder better options that jigging back and forth or simply eating spells and arrows until in range. 
The design was inspired by the sword of Emperor Maximilian I with the motifs adapted to fit the world of Tamriel. The blade features gold etching of the Nine Divines and the sigils of the counties of Cyrodiil from Oblivion, the pommel and quillons inspired by the White-Gold Tower of the Imperial City.
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This is being uploaded in LE as the 2.0 version of the mod. I initially released this for SE/AE unfortunately right when one of the most recent updates hit which caused numerous bugs and compatibility issues that were unfixable. As such this has been rebuilt from the ground up and should work with all versions of the game. I will be releasing an update to the SE/AE page of the mod for those versions.
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Tremendous thanks to @Sarthes Arai, @jadka, @thronplunderand @GammaMetroid in the Arcane university discord for their help in getting
the power attack script running and the special export techniques needed for
the fire mesh.Thank you to Goblinhours69 for allowing me to study and derive from his imbued weapon scripts https://www.nexusmods.com/skyrim/mods/32237.Thank you to Matth85 for his explanation of 1 handed powerattack scripts in this post: https://forums.nexusmods.com/topic/3147589-sword-that-fires-thunderbolt-when-charged-attack/Thank you to Humus for the superb cubemaps and Billyro for converting them for Skyrim https://www.nexusmods.com/skyrim/mods/81213.
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Install: Paste file contents to Skyrim/data folder