You may have already found, and killed Mastrius without even knowing it. That's the tough part about this cheat.

You can find Mastrius fallen, in an ancestral tomb in Red Mountain. You may wonder why he's not talking to you. Its because he only talks to Vampires. Become a vampire (Preferably Aundae, if you're looking for some rare artifacts), and go to his tomb. He'll have a few quests for you. One of them is to retrieve the Shadow Shield from Bthuand, a dwemer ruin far west of Vos. If you've readmy other cheats, you can guess what to do next ;).