Complex sentence: Theoretically, the dog could not have been "killed" since it is part of The Charter but it instead decides to walk off into Death for some time and it can be assumed that Lirael never sees the Dog again.
Split into simpler sentences: Theoretically, the dog could not have been "killed" since she is part of the Charter, but she instead decides to walk off into Death for some time. It can be assumed that Lirael never sees the Dog again.