Alistair Grim's Oddaquaticum by Gregory Funaro

The public believes Alistair Grim is a criminal following the adventures in book one of the series, and Grubb is now an apprentice to his father. Alistair's plan to defeat Prince Nightshade requires finding King Arthur's sword Excalibur, but they must travel to another dimension in order to reach the magical sword. They pass through the secret, underwater entrance to Avalon, but their hopes for success rely on a shaky alliance with a captured banshee. Grubb finds a wall painting that reveals a vague prophecy, and it seems he is more important to Prince Nightshade than anyone expected. However, no one truly knows what makes Grubb so special.

You really need to read the first book in the series before reading this one. Grubb is a thoughtful, caring young twelve-year-old boy, or there abouts, who always ends up at the center of the action. The author continues to drop clues about his mother, who died when he was an infant, but it's still not clear where the story is headed. She appears to have connections to Avalon. I enjoy the relationships of the characters created by the author. Alistair is learning to be a father to Grubb, and Grubb is learning to be a son after living with a mean foster-father for most of his life. The captured banshee is in love with the Odditorium's banshee, but he has a duty to the banshee council to arrest her. I'm looking forward to the next book.