Zelgius, also known as the Black Knight, is a central character in Fire Emblem: Path of Radiance, and its sequel, Radiant Dawn. A former student of the legendary swordsman Greil, Zelgius later kills Greil, beginning a fierce rivalry between himself and Ike. As one of the Four Riders of Daein, he seemingly serves the Mad King Ashnard, but his true allegiance belongs to the corrupt bishop, Sephiran. Zelgius is a steadfast, loyal swordsman who takes pride in his skills, unflinchingly seeking a challenger worthy of providing the duel that Greil could not. As a Branded, he ages slowly than most humans.
Mark Dias plays Zelgius when we see him in Path of Radiance. Here we see him more as the mysterious Black Knight who eludes and torments Ike. He effortlessly portrays the sociopathic demeanour Zelgius provides while killing Greil in one of the game's most memorable scenes, and the cold disappointment he feels regarding Greil's stature as an opponent.
Dave White gets to play him once his identity has been revealed. Though the voice on the surface might seem jarring coming from an unmasked Zelgius, it works excellently for him while he is in full armour. Although Radiant Dawn's voice direction was not always the most polished, you can see the heart of the right idea beneath it all. Both Dias and White's performances provide a cold level of isolation and alienation between Zelgius and those around him that lends his presence a bit of a chilling weight.
However, Robert Clotworthy's portrayal is the most well-rounded on all fronts, I feel. He manages to take the best aspects of Mark Dias's performance and weave them into an interpretation similar to what Dave White laid in a way that befits Zelgius while both masked and unmasked. There is a level of cruelty, bloodlust in a way that is foreboding, maintaining Zelgius's detachment from the messy trail he leaves in his wake. As far as a villain goes, it's a treat to hear him.