Okay, so I recently discovered the underworkings of the Imperial city and have been exploring them today. I found a gate that cannot be picked and needs a key under the bloodworks. I've killed every bandit and vampire and none of them have the key. Can someone please tell me where to find this key?
Thanks in advance

You will unlock it during the course of 'The Ultimate Heist'. It's the last Thieves Guild quest.

sorry, but what underworkings, the sewers?

Yes, it's part of the Imperial sewer system, like where you start the game.

Yeh, its used during the final thieves guild quest.

unless your talking about the other one by that i mean the one on one of the dark brotherhood missions

I cant remember using the bloodworks during the dark brotherhood missons, think i used the sewers but not that particular part.

You could always use the console to unlock it and take a look-see.

quote Drizzt

I've sparred with demons from the Nine Hells themselves, I shall barely break a sweat here today.