Frisk did not release the monsters after fighting Asriel, as Asgore was still alive and had not been killed, Frisk could not leave, a new era of peace began in the underground, until you arrive and discover that to get out of the underground you have Than to kill Asgore, then, everyone will try to stop you, and you will meet new characters, who will fight with you and / or become your friends, but, you choose, if in the end you kill them or not, if you kill Asgore or not , and most importantly, you will make a pacifist or genocidal route