Rimuru arrives a bit too late due to his fight with Hinata and find Shion and kids dead as she was trying to protect them while weakened.
He realize its his responsibility since he unilaterally said he wanted them to befriend human and not to confront them and his subordinates took that a bit too literally. He learn through great sage that, since the barrier is still up, he has his chances to revive the dead due to their souls still being inside the barrier, but can 't revive them for now since he's too weak. He create his own barrier and goes on a HUGE rampage against the army. Because he need a human sacrifice of about 10.000 human souls to ascend as a true demon lord, he goes on a lone slaughter and awaken himself. Some other things happens but I won't go into much details, and after awakening since ascending take some times, he goes into OFF mode and leave Great Sage the control of his body for the resurrection process and "some other """minor""" things I won't spoil".