Glass

Location
In Ancient Valley, in the far bottom left corner, if you can see the Artifact gate, you are on the correct map variant. Climb the chains that lead up (located a few screens to the right) and roughly 3/4 up there will be a small hole in the wall. Jump into it and you can pass through the inner wall. Continue walking left until you drop down a hole. You must press all 5 buttons before the first resets. The first is straight ahead, before the platforms. The second one is in the left wall, before the second set of platforms. The third is underneath, also on the left wall. The fourth is on the right side after a brief drop down. The fifth is the most difficult, as you must jump down and to the left, land on some chains, and space your jumps correctly to make it all the way to the left. You will hear a sound if you press it in time, and a light "click" noise will indicate that the first button has reset already. Once all five buttons are pressed, drop down and the artifact will be in the shrine. Note that this can be done on Drizzle (easy) difficulty, but the artifact might not appear in the character select screen until after restarting the game.