These are the two main characters (and combatants) of The Brothers' War, a historic event from Magic's past, which features in this year's upcoming MTG set of the same name
Being single color means it can go into more decks and even when its not convoked, it has a perfectly reasonable casting cost for exiling any nonland permanent
Surprisingly, AFR only introduces two new mechanics to the game: venture in the dungeon and pack tactics
Of course, we all copied this kind of behavior, leading to numerous illegal plays
Do note that it's legendary card not permanent, so the Captain can even go find legendary sorceries, like Karn's Temporal Sundering or Yawgmoth's Vile Offering
Richard designed these cards himself, including naming each card and commissioning an artist to create artwork