Complex sentence: Casey seeks to befriend "Hedwig", another personality with a preteen mentality, who confides that they will be sacrificed to "The Beast", a fabled 24th personality, and inadvertently reveals that the cellar has recently been reinforced to hold them.
Split into simpler sentences: Casey seeks to befriend the personality with a preteen mentality named "Hedwig", who confides that they will be sacrificed to "The Beast", a fabled 24th personality. Eventually, "Hedwig" inadvertently reveals that the cellar has recently been reinforced to hold them.