I’ll go first. Mine is the instant knockout drug. Like Dexter’s intramuscular injection that causes someone to immediately lose consciousness. Or in the movie Split where there’s the aerosol spray in your face that makes you instantly unconscious. Or pretty much any time someone uses chloroform.
“The mentor/parent has to die so that the hero can prove they’re self-actualized” or whatever. It’s okay for your hero to have living parents, even if their parents are also heroes. I promise your story won’t be less interesting if your character’s mentor figure survives.
In my tabletop RPG campaigns I always make it a point for my characters to have at least one living parent, and usually two. These games are always so full of haunted orphans whose villages were burned to the ground or whatever.
Well adjusted individuals with a good social/familial network rarely become wandering mercenaries, but it’s so refreshing when everyone else is an orphaned lone-wolf prodigy with secret ancestry in the royal family