I need some drogon blood to give too a guy in Umar Hills so he can make me an powerfull armor. So where is it that I can find this dragon blood?

Im sorry but I cant answer your question, but can you please tell me how you found this guy? Where is the book you need to talk with him? Book of Za....... (dont recall the name).?

Talk to him and buy the History of the Zhentarim off him. He will ask you if you've met before. If you answer with the right name, he'll say that he can complete the human skin armour if you get him some silver dragon blood. The silver dragon is in the Underdark, but you're going to have a lot of trouble if you kill her. The human skin armour isn't worth it, if you ask me, and the idea makes me want to retch.

You should kill the silver dragon because it is a dragon and you are superior to it.