S1·E15 The Gorge of Deception!

As they wander further into the Digital World, the DATS stumble upon a ghost city which is home to a nest of Dokugumon. Without warning, the spider Digimon quickly trap the DATS and their partner Digimon, dragging them all into the depths below where their lord MetalPhantomon dwells. He casts the DATS into a world of endless nightmares and despair where their greatest fears take shape. After reliving a painful memory about his mother, Thomas' DNA awakens, granting Gaomon the power to digivolve into his Ultimate form, MachGaogamon.