Immortals Fenyx Rising

This will happen at the beginning of Hephaistos's questline which takes place in The Forgelands, the area in the NE of the main island and will likely be the last of the 4 you play through although you can go right there from the get go if you like a challenge.

Start off by reaching the top of the huge Hephaistos' statue with a hammer in the SW portion of the area and use far sight to open up the map. After the cutscene, check out the noise below the statue near a cave for another cutscene in which the achievement will pop.