Matthew Mercer (@matthewmercer) is a voice actor and a friend of Matt Colville. He is well known for being the Dungeon Master for his own Dungeons & Dragons show, Critical Role.
Matt Colville ran a one-shot for Matt Mercer (alongside Fryda Wolff, Jerry Bennett, Liam O'Brien, and Marisha Ray) as a way to test out his new world. He played Mardun, a tiefling paladin.
The Shield of Gravesford will later meet Mardum and the other characters in the one-shot as non-player characters run by Matt Colville.