Scattered throughout Kazakh are eleven fallen striders, the bodies of Striders who have come before but now perished. These are equipable skins that you can outfit your Strider in. Unfortunately they are the only collectible with achievements related to it that do not appear on your map, which make them hard to find. They look like little 8-bit characters, so when you do find one you won't miss it. Below is a video showing the twelve locations. The great thing about this video is at the start of each Strider a little map will appear showing you the general location in the City. Using this, you can try to guestimate whether to progress to the left or the right of your current area. To see which Fallen Striders you have collected, just bring up the map then use / to scroll through to costumes. Here is a list of all twelve Strider skins, with those you haven't collected stating 'locked'.