Focused Strike
Hold the button to sheathe your weapon and prepare a parry. Release to attack for 500% damage, or 1000% damage if released just before you would take damage, gaining iframes while the attack resolves if timed correctly.
It takes a lot to compete with Zero Mega Man X's favorite circle spin slash move, but here we are, they've done it. Focused Strike is equal parts get-out-of-jail-free card and huge damage spike with a surprisingly large range. The only issue is that it requires you to time it for when an enemy would attack you, but uh... you're playing Risk of Rain Returns. There are fewer frames where enemies aren't attacking you than frames where they are. Oh, and yeah, you do lose the airstalling from Whirlwind you would ordinarily use as a defensive maneuver, and Whirlwind comes out instantly, does 2 hits for extra proc chances, and doesn't need timing, but come on! Parry attacks, in a swarm survival game! You tell me that's not good with a straight face, do it, I dare you.
Unlock: Either clear "Don't Think, Feel" or clear 25 stages.