It is part of a trilogy quest; first go to chorrol and do the "Seperated at Birth" quest, (find the missing twin brother, then meet them again in Chorrol), then do the "Legacy Lost" quest. (Find Weatherleah and kill the 3 ogres).
Go back to Chorrol and wait 3 days, then stand outside. A man will come up to you and ask you to retrieve "something" from the Redguard Valley Cave. It is very near Weatherleah. (The twins father stole it from the Castle, and the ogres took it).
Follow your journal to the cave. 18 ogres, but you can avoid most of them by dropping into the pit in the first big room, go right and follow it all the way through to the very end, a small room with one ogre. Kill him and take the sword from him.
1. Your Journal will tell you to take it to the Castle and turn it in, ignore that, the reward you get isn't worth losing that sword.
2. If you are not in the Thieves Guild, just keep the sword.
3. If you are in the Thieves Guild, but Fathis Ules is not your fence yet : just keep the sword till he is.
4. If you are in the Thieves Guild and Fathis Ules is your fence, or are the guildmaster of the Thieves Guild, save the game and bring it back to the man, he pays you about 1500 for it, don't close out the screen, go to the barter screen and buy it back from him for 2000.
The reason I said save the game first is that he sometimes won't sell it back to you. If he doesn't then just load that save and keep the sword without talking to him, then go back to him when he becomes my fence in the Thieves Guild, or when he will sell it back to you.
5. When you buy the sword back from him your journal tells you to go tell the brothers about their father's sins, DONT DO IT! Ignore your journal again, the brothers will hate you the rest of the game if you tell them about their father. Just keep the quest as unfinished the rest of the game (like you do with Umbra)
The sword is awsome, it has to have some special powers because it is stronger than any other ebony blade. It weighs zero the rest of the game, and you can save money on repairs by doing it yourself. The Honor Blade starts out 10 hit points higher than any other sword, and I have had it grow to 35 hit points. I have had Umbra at 37 when the Honor Blade was at 32, and the Honor Blade killed quicker and cleaner than Umbra. (and it kills undead, daedra, vampires, everything). Even after you buy it back, you still can't drop it, meaning you can't enchant it, but it has never needed it for me. It ROCKS killing anything!
6. Or, you can just keep it.