The Mystery Cave is a location that has a Lamb Relic in the center of it. The cave is one of the many entrances to the Spirit World all over the universe. It is connected to the Ice King's home.
The cave is a large room that contains numerous upward spire-like rock formations that rise to the top of the cave. The cave is dark and has a small pedestal in the center that holds the Lamb Relic that the Ice King placed to trick Finn into the spirit world. This is also the location spirits gain access into the Ice King's home.