Some major bosses that all players must defeat are
-Asylum demon
-Belfry Gargoyles
-Chaos witch Quelaag
– Iron Golem
– Dragon Slayer Ornstein and Executioner Smough
-Seath the Scaleless
– Sif, the Great Grey Wolf
– The Four Kings
– Pinwheel
– Gravelord Nito
-The Bed of Chaos
-Gwyn, Lord of Cinder (and sunlight)