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A big burst of magic that summons monsters and grants super powers at once sounds like a Mass Super-Empowering Event.
Some of these would be Takes One to Kill One, where you're weaponizing the enemy's own species or body parts because nothing else really works.
If two opposed supernatural or extraterrestrial factors are merely using Earth as a stage for their own conflict, then maybe Earth Is a Battlefield. That opens up into one side being Always Chaotic Evil and wanting to destroy everything, and the other side saying "Look, we want to help, but we can't be everywhere at once, so how about we give you some weapons to fight them?"
Edited by KD
Create Your Own Hero and its subtropes Phlebotinum Rebel and Faustian Rebellion could be relevant here. Balance Between Good and Evil, As Long as There Is Evil, and Good Needs Evil could apply if the setting specifies that the presence of evil spirits somehow necessitates/creates good ones.
Evil Will Fail could also be in play, in the sense of "Evil contains the root of its own defeat."
Edited by Unsung
Thank you. It seems Takes One to Kill One would be applicable.
Edited by Orbitron25
What do you call it when the supernatural enemy also brings mankind the means to fight it? (This is used to explain why there are two supernatural factors happening to Earth at once.)
-The magic that brought the monsters also brings super powers to the human defenders.
-You drive klaxosaurs to fight klaxosaurs.
-When spirits appear, humans make deals with good spirits to be strong enough to fight the evil spirits.
-The Cursed Gear are powered by demons, used to fight demons.
-Humans are injected with Aragami DNA to be able to wield new weapons against the Aragami.
Edited by Orbitron25