Forum:Qusetions on cards

I was playing with my friend and he activated Royal Oppression when i special summon stardust dragon assault mode but not to negate my special summon he just activated it. Can you activate Royal Oppression without even paying 800? Can stardust dragon or the assault mode negate Royal Oppression? I know it a continues card but i know i can negate it on it activation not to when it already activated on the field.