Go to Rountabel Palace (Located on Noble District). Enter the palace and follow the knight to a room. Check the shelf beside Mikleo to find a discovery point (Rountabel Palace Bookshelves). Talk to everyone on this room and then follow the knight again.
Follow the assassin, on the small room check the small fountain with no water to unlock the door (there is a rosemary nearby).
T1 = Life Bottle
T2 = Anthem Ribbon Tie
T3 = Lavender (Herb)
T4 = Verbena (Herb)
T5 = ??
T6 = Elven Crown
T7 = Locked
You can shop on Dark Turtlez Emporium. Try to have an equipment that can reduce fire damage (fire ring, etc). There is a discovery point (Ancient Underground Prison) near the turtlez.
Resist: Earth and Wind
Weakness: Physical Attack
Tips: Armatize with Lailah, because Lamia weak against fire. You really need to kill them quickly.
Focus to defeat all Lamia first, before targeting Echidna. Once Echidna’s HP drops below 50%, she will start to cast Pyrogenic Ring. It deals huge fire damage around her, so immediately step back when you see her casting an arte and order everyone to spread out (right analog tilt left).
Once you have defeated her, check the torch nearby and use silver flame to lit it (S1). Continue to S2 and lit another torch. The door to the exit should be opened now. Continue and go back to the inn.