Dragon Slayer Ornstein and Executioner Smough

Dragon Slayer Ornstein and Executioner Smough are a pair of bosses that guard Gwynevere, Princess of Sunlight in Anor Londo.

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

Summoning
It is possible to summon Solaire of Astora to help out in the fight. His summon sign can be found on top of a small platform connected to the stairs to the left, facing the fog door.

Strategies
The player must fight both at once. When one is killed, the other absorbs their dead comrade's power and receives a HP, damage, and resistance boost, as well as recovering all lost HP. They also take on a physical characteristic of their fallen comrade: Ornstein grows in size while Smough is imbued with lightning.

Having a summoned phantom can make the fight significantly easier, as this can cause them to fight separately.

Killing Ornstein first can make the boss fight easier to win, since Smough has more weaknesses than his partner.

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

Charging Spear
Ornstein glides towards the player, and then stabs with his spear. The attack can be blocked or dodged, and a counterattack is possible to achieve.

Lightning Banzai Drop
Only used if Smough was killed first. Ornstein jumps up, pauses mid-air, then falls onto the ground, causing a wide lightning field created from the point of impact. This cannot be blocked, and can only be dodged by keeping one's distance.

Quick Lightning Spear
Ornstein uses his weapon's power to shoot a small burst of lightning at the player. This attack is rarely used.

Heavy Lightning Spear
Ornstein charges up a lightning bolt that he fires at the player. This Lightning Bolt is unusually slow once fired, and has weak homing capabilities.

Lightning Impale
In Ornstein's second form, he will charge his spear with lightning, and then lunge at the player in a quick stabbing motion. If this attack hits an unblocked player, Ornstein will raise the player and release the charged lightning, dealing huge amounts of damage.

Joust
Smough charges at the player with his hammer, then swings it upwards diagonally when he reaches the player. This deals heavy damage, especially when in Lightning-Imbued Form. It can be avoided by running directly away or with the right dodging speed. Blocking will break the player's guard if they are using a shield of inadequate stability.

Swing
Smough swings his weapon. It is possible to roll under this attack and start attacking before Smough has time to recover. Blocking this attack may use a large amount of stamina.

Smash
Smough slowly raises his weapon and then hammers the ground in front of him.

Jumping Hammer
Smough jumps into the air and then hammers the ground as close to the player as he can get. This can be avoided by rolling under Smough while he is airborne.

Banzai Drop
Smough slowly flies into the air and then crashes into the ground below. Smough takes a while to recover after using this attack. The Lightning Form of Banzai Drop also fires out lightning when Smough lands.

Trivia

 * If Ornstein is defeated first, his hand appears to move as if in reaction to Smough's hammer, suggesting he was still alive and that it was Smough who readily gave the final blow, owing to his title as executioner.
 * On the other hand, if Smough is defeated first, Ornstein respectfully places his hand over his body to absorb his power.
 * There's a chance that the Leo Ring will drop from Ornstein regardless of the order in which he is killed.