The Great Grey Wolf Sif is non-optional boss (against what many say), Dark Souls. The player meets Sif in Darkroot Garden and fights with the Greatsword of Artorias in his mouth. Upon defeating Sif, the player gains the Covenant of Artorias, which allows the player to survive the abyss, allowing them to fight The Four Kings.
The soul of Great Grey Wolf Sif can be used to craft the Greatsword of Artorias or the Greatshield of Artorias. To produce a cursed version, that can damage ghosts, of the Greatsword of Artorias the player must ascend a +10 longsword and infuse the Soul of Sif with said sword at The Giant Blacksmith in Anor Londo.
Sif can also be encountered when the player visits Oolacile in the past in the DLC. While exploring the Chasm of the Abyss, a younger version of Alvina will guide the player to an illusory wall. Destroying the wall reveals Sif surrounded by several Humanity Sprites. Sif is protected from the Sprites by a seal created by Knight Artorias with his shield. Eliminating the Sprites will allow the player to collect the shield and free Sif, who can then be summoned to aid in the battle against Manus.
Should the player free Sif and summon him in the battle against Manus, an alternate cutscene will be played before the Sif boss battle in which Sif clearly recognizes the player but still reluctantly takes up arms against him.