Complex sentence: Before they depart, Gibbles gives them magic cards that perform spells just by reading the inscription on each one; however, they have to be saved for emergencies because each card can only be used once and each spell is temporary.
Split into simpler sentences: Before they depart, Gibbles gives them magic cards that perform spells just by reading the inscription on each one. However, they have to be saved for emergencies because each card can only be used once and each spell is temporary.