Ancient Fabric is a block added by the Dimensional Doors mod. Ancient Fabric sometimes appears in Limbo. When standing on it while in Limbo, the player will appear in overworld with some Unraveled Fabric around if the player has spawned on the ground. If ancient fabric is in the limbo it will go through a phase of changes, first turning into stone, then cobblestone, then gravel, then unraveled fabric. It sometimes skips from stone or cobblestone to unraveled fabric. The whole process takes anywhere between 3 minutes and 21 minutes.
Ancient Fabric can be used to craft Stable Fabric. It can also be used to make traps by putting it on the ceiling and letting it turn into gravel over time to fall down and suffocate mobs below.