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Deals damage equal to the user's level. Ghost-type Pokemon are immune, all other Pokemon take regular damage.
At first it looks like Seismic Toss just doesn't hit hard enough, considering most competitive matches are played at level 100. However, a closer look shows that it has a specific niche, the fact that it does fixed damage means that it will bypass any defensive boost that the opponent may have garnered and will hit for the same amount of damage regardless of resistances (it won't bypass immunities, though). All of these perks are great news for most walls since they usually don't have the necessary power to break through defensive cores. The 100 fixed damage also means that said walls will be able to break the majority of the Substitutes, thus they will not become an invitation for set-up sweepers or Taunt users that are looking for a free turn. With all that in mind the most prominent users are Chansey and Registeel as they have pitiful offenses and can't make use of any other offensive move option to deal damage.
Night Shade is a Ghost-type equivalent.