Defeat Gluttony.

Story-related; unmissable.

Combat tips: Flame Hollow Salvation charge is key here. When Gluttony starts spitting acid at you, run towards him and run around him. This will cause his spits to be cut short. When he tries to inhale you, rapidly spam 'RB' towards the screen; that's how you'll avoid those attacks. Now, when he starts to whip his tentacle at you, head towards the nearest wall, his reach will not hit you. Charge your Flame Hollow Salvation as when the tentacle stops, toss it and hold 'X' for a nice combo attack. Once he reaches 50% health, the battle moves under water.

Once under water, swim towards the homing mines on both sides of the area, and get close enough for them to follow you. Then swim towards Gluttony so that it'll start to inhale you. This way it'll swallow the mine. Only one mine is necessary as it causes the same damage if it inhales two of them but bait both to him in case he doesn't inhale one. When he is down, swim up to him and press 'B'.