There are others that have added this weapon to the game but I felt it was not done properly and sometimes overpowered. This mod gives it the same enchantment as the one in Morrowind and is a powerful dagger, but I feel like it is balanced and usefull at the same time. All of it's attributes were pre-assigned by Bethesda.
This mod will replace the silver dagger (at same original location of the Mysterium Xarxes) inside the Dagon shrine with the Razor. So if you have done the main quest just sneak back into the shrine and get it. If you haven't done the main quest then you have a chance to grab it when you infiltrate the shrine. I also gave poor Jeelius a fighting chance; I upped his health and gave him a restore spell.
I also included a file that makes Umbra a removable item if you are using the sword outside of it's quest. Normally most wouldn't bother removing it but I like to not carry it at times.