The Dark Portal (The Deptford Mice Trilogy‪)‬

Every mouse knows: Beware the Grill.

On the other side of the Grill lie the sewers, where bloodthirsty gangs of rats roam the tunnels. It is in the depths of this blackness that the mysterious god Jupiter has his lair.

But one curious mouse can't resist the perilous world beyond the Grill, and soon no mouse can ignore its dangers any longer. Before the alter of Jupiter himself, a small band of mice must face evil at its strongest, or the entire city of London could be destroyed.