The entrance to this place is located in Ashina Reservoir, from the same cave where you first started during the prologue. To access this, you have to defeat the Lone Shadow Longswordsman, guarding the entrance to the cavern leading to an Abandoned Dungeon.
After defeating the said boss, continue to the flooded tunnel and grapple your way up. Carefully continue to the large cavern, where two geckos are waiting in the dark. Be careful since there’s a headless waiting at the bottom. To reach the Sculptor's Idol here, you have to do a running jump across the ledge to avoid falling near the terror-inducing mini-boss.
You don't have to explore this area fully early on but you can at least commune with the Sculptor's Idol so you can travel to this place later if you want.
To continue to the depths, continue to the tunnel past the sculptor’s idol and jump off the shrine gate to the abyss waiting below. Make sure to grapple once you see a green dot while you’re in mid-fall.