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 a 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 (none of whom appear in game), Dragon Slayer Ornstein was one of Gwyn, Lord of Cinder's four knights.

Statistics
Normal Game: 1,642/2,982 HP (Ornstein) 1642/2982 HP (Smough), 30,000 Souls New Game+: ?/? HP (Ornstein) ?/? (Smough), ? Souls

Attacks
Smough:

Ornstein:

Electric Smough:
 * Horizontal Swing - Smough performs one large swing with his hammer. Easy enough to dodge.
 * Vertical Swing - Smough smashes down with his hammer. Again not too hard to avoid (roll out of the way if you must). Smough takes a while to recover so you can sneak in an attack if you're quick.
 * Charge - Smough runs up then flicks with his hammer. Best plan of action is to back away, as he can catch you even if you roll to the side. He again takes a while to recover so you can try and sneak in a hit or two.
 * Slam - Smough jumps up in the air and slams down on the ground, releasing a large electric field. Smough's most annoying attack, can deal large damage if caught. If you see Smough rising the best course of action is to roll away as quickly as possible.

Giant Ornstein:

Drops
Upon killing the pair you obtain either a Soul of Ornstein or a Soul of Smough (you get the soul of whichever one you kill second). If you kill Ornstein first he drops a Leo Ring. Domnhall of Zena will also sell the armour of whichever one you killed second.

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
As might be expected the pair are one of the most annoying fights in the game. Having at least one phantom with you (even if it is just Solaire) can make a big difference especially when fighting both of them.

The fight itself can be thought of as having two parts, the first being where you fight the two together, the second where you fight the buffed remaining one.

Depending on which you kill first the fight can be considerably easier. Of the two Smough is generally regarded as the weaker. He has lower resistances, is slower and all of his attacks can easily be blocked. Orstein on the other hand is quicker, has higher resistances and some of his attacks deals lightning damage.

Because of this most find it an easier strategy to work on killing Ornstein first, as while its harder a buffed Smough is less of a threat in the long run. Having a phantom with you at this point to distract Smough makes things a lot easier, as it means you can focus on dealing with Ornstein. Even if you don't have a phantom, trying to keep Smough away while you deal with Ornstein is the best route of action.

Assuming you can keep Smough away dealing with Ornstein isn't too hard, just be wary of his lightning attacks and you should manage.

Once Ornstein is dead however things hot up. While his attacks are exactly the same as before they now carry lightning damage, meaning he can wear your HP down even if you're blocking.

Because of this holding off Smough at this stage can be a real pain. All of his attacks however take a while for him to recover from, so if you're a close combat fighter you should take your chance when he's recovering and smack him.