The Hired Thug's Missive is a note found in Labyrinthian. It speaks of the origins of the Wooden Mask being found in Labyrinthian, in the form of the story of two unknown mercenaries tasked with protecting a Breton scholar. The Breton took the two mercenaries to Labyrinthian, where he astonished them with his use of the Wooden Mask in order to time travel.
Towards the end of the journey, the mercenary grew tired of waiting for the scholar and promised that if he showed up again he would "put this dagger through his chest to keep him in one place." This proved true as the skeleton found with the mask can be found with an Orcish Dagger through its ribcage.
In connection with Captain Valmir (found at Forelhost) and a note on his corpse, it seems the Breton was a scholar or mage hired by the Thalmor to find the Dragon Priest masks, or at the very least attempt to understand the mystery surrounding the Labyrinthians ruins. By the Breton's corpse and Valmir's demise by the Dragonborn, it can be assumed this mission was a total failure.
It seemed like an easy enough job. Groz and me done plenty like it before. Some chinless Breton wants bodyguards for a trip into the mountains. Fine. Easy clink for us.
He goes on about this twice-forsaken mask of his the whole way, of course. Got a pretty good laugh when Groz snatched it and put it on her face - he threatened to fire us for that one. Not smart out here on your own, he realized that right fast and shut up about not paying.
So we get here, he thumbs through some papers and mutters to himself - never you mind that me and Groz had to cut through ten stinking trolls just to get him here - and without so much as a warning, poof. He's put on that mask and vanished. Could have put my hands on his throat one moment, the next he's not so much as thin air.
Well, after a while we didn't know what to do, and Groz picks her gear up to head home, when he poofs back, that mask in his hand. Begs us not to leave, says he needs us to wait, this is what he's paying us for. Then puts the mask back on his sorry face and he's gone again. I'd seen cloaks do invisibility before, but a few swings of my fist proved he wasn't pulling that one. Nothing there to hit.
He shows up again, tells us he just needs more time. Has to figure out something about other masks, and vanishes again. That was yesterday, and I'm done with twiddling my thumbs and writing letters to myself. We're leaving come sunrise, and if he shows up again I promise Azura I'm putting this dagger through his chest to keep him in one place. We'll pull more loot off him dead than he could have paid alive. But not the mask. That cursed thing can stay and rot with him and the trolls right here.
- The two mercenaries were most likely Orsimer, supported by the fact that one of them was named Groz, and that an Orcish dagger was used to kill the scholar.
- At least one of the mercenaries might have been Dunmer, due to the phrase "I promise Azura."