Final answer:
In the story of Percy Jackson, aside from Zeus's lightning bolt, c) Hades's Helm of Darkness was also stolen, causing a potential war among the gods.
Step-by-step explanation:
From Hades, Percy learns that not only the lightning bolt was stolen but also another extremely powerful and magical item, which was the c) Helm of Darkness.
In Percy Jackson's story, the Helm of Darkness is the property of Hades, the god of the underworld, which grants him the power of invisibility.
This item's theft is just as serious as the theft of Zeus's lightning bolt, creating turmoil among the gods and a potential war amongst them which Percy must prevent.