Hades and the Helm of Darkness (Heroes in Training Book 3)
The Heroes in Training are entering the Underworld—if Hades can conquer his canine fears, that is. The Underworld usually isn’t really meant to be a fun place—but tell that to Hades! He loves the dark and the stinky smell of sulfur. However, there is one thing that Hades is not a fan of: dogs. And when Zeus and his fellow Olympians encounter Cerberus—a snarling, three-headed dog—Hades must conquer his fears and tame the hound so everyone can continue into the Underworld and deposit their Titan prisoner, Oceanus, back where he belongs! But with magical water that causes forgetfulness, hot beds of lava, and another epic battle with two more Titans standing in their way, will Zeus and his heroes make it out of the Underworld with everyone intact?
Heroes in Training by Joan Holub & Suzanne Williams