The Skull Knight (Skull Knight Lord in Japan) is a boss version of the Skull Swordsman. He first appeared as the boss of the first stage in Castlevania III: Dracula's Curse, and later returned in a much more powerful portrayal in Harmony of Dissonance. He appears to be the remains of an ancient dragon-like warrior who comes armed with sword and shield, and is clad in armor.
The Skull Knight is the boss of the first stage in Dracula's Curse, the town of Warakiya. He can be found in a graveyard at the end of the level, where he first appears only as a pile of old bones. However, when Trevor Belmont approaches, he suddenly comes back to life and reassembles himself to confront the hero. His only attack is a quick downward slash, but he can also damage Trevor by touching him, and this is what he'll try to do, by walking steadily toward him, only stopping every once in a while to slash with his sword. He has a considerable amount of HP, but a few flasks of Holy Water should be enough to defeat him in just a matter of seconds.
A stronger version, the Skull Knight King, appears as the boss of the Caves later in the game; the only notorious difference being that now he will also chuck a series of large bones at the player.
The Skull Knight reappears in Harmony of Dissonance as the boss of the Skeleton Cave in Castle B. His boss fight consists of three phases, which are triggered as he sustains damage, and each of which where he changes his combat style:
In his first form, he is armed with sword and shield, and attacks by performing short lunges, attempting to stab Juste with his blade.
After losing enough HP, he falls to pieces but soon regenerates from behind his shield and drops his sword. Now, he mainly attacks by dashing at great speed across the room.
After losing more HP, he repeats his rebuilding sequence, this time dropping his shield. In this final form, he can shoot a large laser beam from inside chest that travels across the entire room.
It's worth noting that in this incarnation, instead of being the remains of a dragon-like creature, the Skull Knight is now the skeleton of a huge warrior who wears a helmet fashioned from what appears to be a bovine's skull.
Curiously, in the background of the boss room, there are two groups of giant armor knights equipped with swords and shields, but the second slot of the left group is empty. It is unknown whether the missing knight is this boss (although the shield design is different) or one of the other giant armor knights in the game.