An Degenerate Serpent-man Champion is a World Boss that may, or may not drop a Skeleton Key that can be used to open a Legendary Chest, depending on where you find him:
If in the wild, Degenerate Serpent-man Champion will be a three skull world boss and drop a Skeleton Key. Several Legendary Chests can be found in each Elder Vault or around The Spire.
Skeleton Key that can be used to open a Legendary Chest, depending on where you find him: