You'll find this during The Legend of Red Eagle in the Sundered Towers.
This quest is initiated once you read one of the Books of the same name. The book can be found in a few different places; one location is in Solitude.
Objectives
Find the Red Eagle’s Sword
Unlock the secret of Red Eagle’s Tomb
Walkthrough
This quest begins as soon as you read a book entitled The Legend of Red Eagle. This book can be found in several places, but you can get it early in the game when your presence isrequested at Dragonsreach in Whiterun. After being introduced to Farenger, head to the room to the left of his laboratory. On one of the book shelves a book titled The Legend of Red Eagle will open up this quest.
Travel West to the location marked on your map Red Eagle Redoubt, fight through the Red Eagle ascent. Look out for traps along the path; crafty adventurers should be able to turn traps against enemies. Eventually, you’ll emerge near the Sunder Towers. A Forsword Briarheart has the sword in his possession, remove it with extreme prejudice. Acquire the Red Eagle’s Fury and Red Eagle Tower key and head out towards the towers.
The Bad-assery Of Fire Weapons
Enter the first tower and head towards the top, use the chain to lower a bridge connecting two towers. You now have a clear path to your next destination - Rebel’s Cairn. Head inside and place the Red Eagle’s Fury on the switch to open up a secret tomb. The Red Eagle himself will reanimate alongside some common skeletal warriors. Dispatch the weaker enemies first and beware the Red Eagle’s ability to disarm you. Defeat the undead warrior to claim a very powerful weapon - Honed Ancient Nord Greatsword of Burning. Don’t forget to grab the sword sitting on the switch on the way out, you’ll notice its now changed into the Red Eagle’s Bane.
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