OPTIONAL: SANCTUARY GARDEN (NEW CONTENT!)
Go to where you fought the Hydra in Darkroot Basin. More specifically, where you found the Golden Golem and freed Dusk. If you have the Broken Pendant in your possession from the Duke's Archives, there will be a black portal. Examine it to travel to Sanctuary Garden.
Rest up at the bonfire, because you’ve got a boss fight dead ahead.
BOSS – SANCTUARY GUARDIAN
Go into this battle with light enough armor so that you can do the fastest roll. You want to be able to dodge most of the boss’ attacks as opposed to blocking them. Watch out for the standard melee strikes (just backpedal to avoid), a forward shockwave, a diving attack, and lightning bolts (the latter three of which can be avoided by simply rolling to the side). He also does a single uppercut move designed to break your defenses. If you can dodge to the side after any of his lunging attacks or his shockwave attack, you can hit him while he recovers.
When airborne, the Guardian likes to spit lightning. Be ready with a shield that has good lightning defense (Eagle Shield or Effigy Shield) or simply roll to avoid the projectiles.
Getting struck by the Guardian’s tail means instant poison. If you cut off the tail, you'll be awarded a Guardian Tail whip. The best time to strike the tail is after he does his diving attack (see video). Proceed to the other side of the lake bed after defeating the boss to get to the next area.