You think about why you're here now...
You can feel the empty space in your inventory get smaller and smaller!
Acquiring Last Dream
Last Dream is a consumable item obtained by using the "Dream" ACT during the fight against Asriel Dreemurr.
When obtained, Last Dreams fill all the inventory's free space. This action can be repeated indefinitely, solidifying the impossibility to lose the fight.
Last Dreams heal 17 HP, but are described to heal 12 HP. In contrast, saving an already-saved character during the second half of the Asriel fight heals the protagonist fully.