Coming soon but ill give explanation

If you have ever watched ben 10 not the reboot but the original series then heres the description by the end of s2 i think

So you know how ben unlocked master control at Niagara falls well lets say ben never took off the omnitrix while he was at the null void instead ben managed to get gwen back but since it costed them alot of time Kevin and vilgax were on their tales ben transformed into fourarms and threw gwen into the closing portal then before ben could make it the portal closed leaving ben in the null void with vilgax and kevin but he managed to fight them off and escape