Mermin Volume 4: Into Atlantis

It's all-out war between Mer and Atlantis! At least that's what Mermin's
father, King Merus, declares after Pete is taken by Atlanteans. And that's not
all - Mermin will be fighting side-by-side with his father, so it's time
for some intense training! Meanwhile, deep in the Kingdom of Atlantis, Pete
meets other children without homes who, of course, cannot fathom that he is from
the surface. His stories of Mer attract the attention of another child, who
offers to introduce Pete to her "friends" - who he never expected to see
in Atlantis!