If I have a Tera Pokemon on my Bench when I use Landorus' "Earthquake" attack, do they take 10 damage or not?
No, the Tera Trait protects them from damage by your own attacks as well.
Earthquake





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Rock CannonFlip a coin until you get tails. This attack does 30 damage times the number of heads.


EarthquakeDoes 10 damage to each of your Benched Pokémon. (Don't apply Weakness and Resistance for Benched Pokémon.)
If I have a Tera Pokemon on my Bench when I use Landorus' "Earthquake" attack, do they take 10 damage or not?
No, the Tera Trait protects them from damage by your own attacks as well.