Advanced format ruling
Core Blast
Card effect (current official text)You can only activate this card's effect while you control a face-up "Koa'ki Meiru" monster. Once per turn, during your Standby Phase, if your opponent controls more monsters than you do, you can destroy a number of cards your opponent controls so that your opponent will control the same number of cards as you control monsters.
Current rulings for this card are maintained by Konami.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Advanced forum — sourced change requests get reviewed.