If Frisk absorbed Toriel’s soul to pass through the Barrier, what happened to Toriel’s soul? Did it shatter when Frisk went through the barrier or when Frisk died?
Toriel kept Frisk alive throughout her pregnancy, in the absence of Sans’ assistance. Her soul was expended just before Aliza’s birth, her residual magic expired from the tireless work of sustaining a sorcerer’s demands for magic. It wasn’t enough to spare Frisk, unfortunately.