im kinda new to homestuck and i'm a bit confused, does defeating the black king "stop" the reckoning from happening or is the whole war thing the reckoning itself? and does defeating the black king mean that you can still progress in the sburb game or do you have to scratch and go back?

The Reckoning is initiated when the Black Army defeats the White Army. Once the Reckoning is initiated the players have 24 hours to defeat the Black King before Skaia is destroyed by meteors and they lose. The Black King is sort of like the endgame boss.