Core Overclock (TSHD-EN087) - The Shining Darkness 1st Edition

All face-up Koa'ki Meiru monsters you control gain 500 ATK. Once per turn, during your Main Phase, you can discard 1 Iron Core of Koa'ki Meiru to have all face-up Koa'ki Meiru monsters you control gain 1000 ATK until the End Phase.