Dragon Slayer Ornstein and Executioner Smough

 are a pair of bosses found in Anor Londo. Players must defeat them in order to reach Gwynevere, Princess of Sunlight.

The player must fight both at once. When one is killed the remaining one absorbs their dead comrade and receives an HP, damage and resistance boost (Ornstein grows in size while Smough becomes electric), as well as recovering all lost HP.

Alongside Hawkeye Gough, Artorias the Abysswalker and Lord's Blade Ciaran (all of whom appear in the DLC), Dragon Slayer Ornstein was one of Gwyn, Lord of Cinder's four knights.

Summoning
It is possible to summon Solaire of Astora to help out in the fight. His summon sign can be found at the top of the flight of stairs just before the white light gate.

Strategies
Having a summon can make the fight significantly easier. This is because Ornstein and Smough will be separated from each other so they can't help one another.

Killing Ornstein first can make the fight easier to win.

Swinging Combo
Ornstein swings his spear up to four times in a row. Can be blocked.

Jumping Attack
Ornstein jumps up, and then either stabs at the player, or smashes his spear into the ground where the player is standing. Both deal the same amount of damage, and it can be blocked.

Gliding Stab
Ornstein glides towards the player, and then stabs with his spear. Can be blocked or dodged, and a counterattack is possible to achieve.

Electric Bonzai Drop
Only used if Smough was killed first. Ornstein jumps up, pauses, then falls down to the ground, causing a large field of lightning to spread from the point of impact. Cannot be blocked, and can only be dodged by running backwards.

Lightning Bolt
Ornstein charges up a lightning bolt, then fires it at the player. Can be dodged by running to the side, or hiding behind one of the many pillars in the arena.

Lightning Impale
Ornstein only uses this attack when Smough is dead. He charges his spear with lightning, then stabs out at the player. If it hits an unblocked player, Ornstein then raises the player, and releases the charged lightning. Deals heavy damage.

Joust (Charge Attack)
Smough charges at the player, then performs a diagonal upwards slash when he stops. Deals heavy damage, especially when in Electric Form.