i feel pretty guilty for spoiling deltarune... TWICE (reddit replied failed) but im going to say this in the least spoilery way possible while also giving you an answer that doesnt sound completely insane, since UNDERTALE and DELTARUNE chapter 2 are incredible games id highly recommend playing. but if you really wanna know, >!at the end of chapter 1 of deltarune, the "silent protagonist" of the game, kris, which deltarune mostly focuses on (e.g how the heck would you feel if you woke up the next morning, puppet of an eldritch horror using you as their character for their "game") rips their "SOUL" which in both undertale and deltarune is represented as a heart out of their chest and traps it in a bird cage and does something im not going to spoil.!< again, deltarune and undertale are both 10/10 games, with the best OSTs on a video game, ever, so i'd really recommend playing them. if you are going to, you should play undertale first! go into it blind!