The Secret Room is an unmarked puzzle on Dragonspine that involves the use of the items Princess' Box, Priest's Box and Scribe's Box. Once all items have been found, they can be used to unlock the Secret Room located above the word "Nail" in "Skyfrost Nail" on the map. For more detailed information on where to find each item, see each respective pages.
Located above the word "Nail" in "Skyfrost Nail" on the map, most easily accessed by heading southeast from the Statue of The Seven in Dragonspine and gliding along the mountain.
It is found in a Luxurious chest obtained by clearing a challenge activated by a sword in the ground beneath a tree on a small island east of the Frostbearing Tree. The challenge has two Cryo Abyss Mages and one Hydro Abyss Mage, who you have to defeat within 60s.
It is found in a Luxurious chest on top of a tower, at the leftmost bend of the route above Entombed City - Ancient Palace.
Inside the Secret Room is one Luxourius chest (including a Crimson Agate), one Precious chest, a half-buried out of sight exquisite chest on the ground near the entrance, and an ancient carving for the quest Ancient Carvings.