Thundering Raiju
Creature — Spirit
Haste
Whenever this creature attacks, put a +1/+1 counter on target creature you control. Then this creature deals X damage to each opponent, where X is the number of modified creatures you control other than this creature. (Equipment, Auras you control, and counters are modifications.)
Illustrated by
Kamigawa: Neon Dynasty#389RareEnglishLegality
Rulings
An Aura controlled by an opponent does not cause a creature you control to be modified.
A creature with a counter on it is considered modified no matter what kind of counter it is or which player put it on that creature.
A creature that is equipped is considered modified no matter who controls the Equipment that's attached to it.
Only creatures can be modified. If a modified creature stops being a creature, it is no longer considered modified.