Hellhounds are children of the goddess Nyx and Cerberus, the guard dog of the underworld. Hellhounds are feared by half-bloods (safe for children of Hades due to them being creatures of the Underworld). Hellhounds are very large, tall, and have the ability to travel through shadows. Percy first experienced shadow travelling, done by Nico, when he and Mrs.O'Leary went to Westport, Connecticut, May Castellan's hometown. It takes a lot of energy for hellhounds to shadow travel, especially the first couple of times they use it, and they usually must rest afterwards. Most hellhounds can shadow travel, as shown in The Last Olympian. Through the Mist, Mrs. O'Leary was seen as a poodle (The Demigod Files).
Just after Percy's first capture the flag game, he is attacked and almost killed by a hellhound, which is defeated by arrows fired by Chiron. It is later revealed that Luke Castellan, who was working for Kronos at the time, had summoned it since no monster can enter within the camp's borders unless given permission.
Two hellhounds were seen on the Princess Andromeda.
The main hellhound that appears is Mrs. O'Leary, the only friendly hellhound. She was probably trained by Quintus, because she is very loyal to him and is very sad when he dies. Her bark is said to be "slightly louder than an artillery gun." Quintus claims that he and Mrs. O'Leary became friends through "many close calls with death and quite a few giant chew toys".
Mrs. O'Leary appears when Percy shadow travels for the first time, after Nico teaches him how to do it. During the final battle, numerous hellhounds are a part of Kronos' army. They mainly shadow travel behind the demigods' lines.
Hellhounds are usually described as being large, black mastiffs with glowing red eyes. The average size of hellhounds are about the size of a grizzly bear or a rhino, but can be as large as a garbage truck (e.g. Mrs. O'Leary). However, the film version made them smaller, but more ferocious, having flat heads and with larger, reptilian-like teeth.
- Shadow Travel
- Stygian Ice Whistle: It is unsure if it can be used to call any hellhound, but when Percy blew into it, only Mrs. O'Leary came. This might have been due to Kronos preventing them from answering the call or it might be specifically made for a certain hellhound (most likely the latter).
In the film, Percy does not encounter a hellhound in the forest, but is instead visited by Hades at the bonfire. Later in the Underworld when they enter his palace, they are 'greeted' by angry hellhounds, but Persephone scolds them and they leave. She tells the demigods that the hellhounds sense the presence of another animal, indicating Grover, a satyr. In the film, they resemble gargoyles, having a grayish lion like face and fur.