User blog comment:Luckystarz103/Prepare to Die DLC surprises/@comment-1687225-20121213143612

Sif was Artorias's traveling companion. The two journeyed into the Chasm of the Abyss together to fight Manus, but were overcome. Artorias used his cleansing shield to protect Sif before being taken by Manus.

What's particularly interesting is that Oolacile is heavily implied to be pre-modern Darkroot Forest. I mean, who would move Artorias's grave? The hollows? We get hints of it from the ruined towers that litter the Darkroot Garden, and by the fact that Dusk is summonable there. The weapons that litter Artorias's grave probably belong to adventurers that, like the player, attempted to reach it. Unlike the player, they probably all got killed by Sif.

Sif does not have to be summoned in order for you to access the new cutscene; you just have to rescue him. However, he is also still summonable during the fight even if you already have two phantoms summoned.