Rhoy's Resting Place
Enter the misty door (safe) and kill the mushroom insects. There's a bonfire to the left so light it up and rest if necessary. To continue, kill the large mushroom insect blocking the path. There are more of those annoying mages along the way as well as lurking swamp creatures. You can always fall back to a pillar for cover, especially if you need a breather or time to heal.
After getting rid of the two mages, turn around and follow the narrow path along the rock wall to find a corpse containing a Large Soul of a Proud Knight and a Green Blossom. Loot the corpses here to get Skeptic's Spice, Soul of a Brave Warrior and Petrified Dragon Bone, Red Iron Twinblade and Soul of a Proud Knight, and Homing Crystal Soulmass.
Just take note of the yellow glitters the submerged swamp creatures are emitting to avoid getting ambushed. A dark invader, Peculiar Kindalur will also appear in this area. He's casts fast, poweful soul spears so it will be better to fall back to a pillar and circle around so you can have somewhere to block his magic attacks and allow you to easily deal damage to him. Be careful as well since there's another mage along the ruined path that can snipe you if you get within its line of fire.
Continue along the bridge ruins and loot the Large Soul of a Proud Knight and Green Blossom from the corpse in the end. Continue to the next building. If you talk to the NPC there, she will disappear. Downstairs lies the misty door leading to another boss.
Boss: Demon of Song
This is a large frog enemy and the only vulnerable part of it is its face. If you're a ranged character, you just need to approach it to force it to reveal its face and arms then quickly move back to avoid its flailing arms. If you attack it from a distance, it will dash towards you which is really hard to avoid and will cause tremendous damage. Aside from its normal flail attack, it will stand up and will attempt to crush you. For melee characters, you need to time your attack right after the boss misses its melee attack. This boss is slow and its attack patterns are quite predictable so this battle isn't that hard overall.
After defeating the Demon, continue along the next path. Continue past the passage and head to the tower. You'll find another bonfire inside so light it up and rest as necessary. The door at the top of the tower is locked so ignore it for now. Leave the tower and go through the next path. Open the door and ride the large lift inside. Once you reached the bottom, continue forth to reach the next area.
Key to the Embedded
Since you have the Key to the Embedded, you can travel to Central Drangleic bonfire and use the lift. On the top, use the key to open the cage. Talk to the woman inside to get a Ring of the Undead.