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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Raging CrashThis attack does 10 damage for each damage counter on all of your Benched Pokémon.



EarthquakeThis attack also does 20 damage to each of your Benched Pokémon. (Don't apply Weakness and Resistance for Benched Pokémon.)
Its body can’t be harmed by any sort of attack, so it is very eager to make challenges against enemies.
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.