"This stone monument has survived 10,000 years. I wonder what secret it holds."
Stone Monuments are structures which appear in Breath of the Wild, as a part of the second DLC expansion, The Champions' Ballad. There are four in total, and each have three pillars, which detail a location to assist Link in completing the Divine Beast Tamer's Trial. These locations only display on the pillars when Link nears for the first time, as he is the Goddess Hylia's chosen hero.
Kass can be found near these monuments, playing his accordion. He speaks of his teacher's unfinished song, and plays varying parts to Link at each different monument. The parts list three trials for Link to complete, each leading to a new Shrine of Trials.