S1·E12 End of the Rainbow

An unknown female alien summons another Demaaga to fight X, and times the fight to coincide with a burst of Dark Thunder energy, which strengthens Demaaga into Tsurugi Demaaga, but weakens X to the point that X disappears, after forcing Daichi out of the fusion. Daichi must resort to a dangerous method to save X, and in the process, remembers a moment of his past with his parents, resulting in an upgraded form for X.