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This move always deals 40 HP damage. Dragon Rage isn't affected by STAB, weakness, resistance, or any other factors.
Unlike Seismic Toss and Night Shade, which can hit for up to 100 damage, Dragon Rage does a fixed 40 damage per use. This only makes it useful in Little Cup, where the low amounts of HP make it capable of easy OHKOs or 2HKOs; however, because it is almost completely uncounterable, it's banned there, and therefore is not viable competitively.
Night Shade is a Ghost-type variant that always deals an amount of damage equal to the user's level.
Seismic Toss is a Fighting-type variant that always deals an amount of damage equal to the user's level.