http://www.youtube.com/watch?v=3-B8P5JEGuY

where can i find the key?

idk, but the notes on the video mention that you need a key from the jarl, gotta try it out with my thief

Its connected to a quest. You have to be at least level 20 to begin the quest. To do so talk to Hulda (?) in The Bannered Mare

This.

Its a pretty decent blade too. I use it because its 2h, but also swings much faster than greatsword, axe, or hammer.

Please do not post Spoilers in General.

People come to General to learn about the game without learning about what happens in the game. If they want to know about what happens, that is what the Cheats, Hints and Spoilers section is for.

Do not use quest details or location specific details related to quests/puzzles/NPCs in your posts in this section. General is for discussing gameplay and mechanics and general topics about the the game including but not limited to races, items and their use, magic, enchanting, skills, character builds, criticism, praise, funny random things that happen, places from an esthetic standpoint and and other general topics.

Use the Spoilers Section if you have to post quest details or other spoilers or a thread about the same. I will move this now.

It's not really hidden or secret. It's part of a Daedric quest called "http://www.uesp.net/wiki/Skyrim:The_Whispering_Door". Ask Hulda at the Inn about rumors, that'll get it started if you have a high enough level.

And to save you the trouble of trying to steal the key without the quest- as far as I know, the key doesn't spawn without it.