Heroes in Training -Crius and the Night of Fright #9 by Joan Holub and Suzanne Williams*

My eight year old loves this series.  He read this book himself.  This is his review.
Zeus and the Olympians went into a tower that was surrounded by darkness.  It was always night.  They were trying to find Artemis, Apollo's sister.  They had to beat Crius, a Titan who calls himself "the highest" and had to defeat Pandi, Crius' guards.
*Highly recommend.