Abridged requirements:
Get all three endings (requires three full playthroughs)
Find all 107 rings, all 35 Miracles, all 34 Sorceries, all 27 Pyromancies, and all 33 gestures
Beat all 18 bosses (including optional bosses)
Discover all eight covenants
Upgrade a weapon to +10
Infuse weapons using the 15 different infusion gems
Fully upgrade the Estus Flask
Fully upgrade the Firelink Shrine bonfire
Reach the two secret areas