Complex sentence: All three boys survive the night, with Thomas successfully killing a Griever by tricking it into running into a closing wall, squashing the griever, and they return the next day with Alby to the astonishment of the other boys.
Split into simpler sentences: All three boys survive the night, with Thomas successfully killing a Griever by tricking it into running into a closing wall and squashing the Griever. They return the next day with Alby to the astonishment of the other boys.