The Map of Heaven is a map that is obtained by defeating Lelindre with Siris. It does not yield an item, instead it reveals a secret part of the Metius Observatory. Along with the Map of Heaven, you need to acquire the Blade of Heaven and the Axe of Heaven and equip them to acess the hidden pathway.
The hidden location is where Varun ,Watcher of Heaven, will be found.
Winning the fight against Varun will reward you with The Origin in a chest, which is the Worker of Secret's (spare) suit of armor.