Shadow Travel

Shadow Travel is a form of transportation that allows creatures of the Underworld and demigod children of Hades to travel to any desired place on Earth or in the Underworld, although it makes the user extremely fatigued.

Shadow Traveling
Hellhounds and children of Hades use this traveling method by slipping into the shadows and coming out somewhere else of their desire. It is unknown if children of Pluto use shadow travel or not, but most likely they do. Exactly what happens in the shadows is not known. It is to be assumed that it does not simply take them there, but acts as a tunnel, as when Mrs. O'Leary shadow traveled she was still running after they got into the shadows when Percy Jackson and Nico di Angelo were traveling to Westport, Connecticut to find May Castellan. Shadow traveling exhausts the user greatly; for example, Nico said that he passed out for a week in China when he first tried it, and Mrs. O'Leary herself was tired after doing it. Percy describes how it feels to Shadow Travel as "going so fast that it feels like your face is peeling off."

The Sword of Hades
Persephone sent a shadow to Nico's location and apparently, forcefully shadow traveled him to New York to join Percy and Thalia Grace. This was not mentioned as being shadow travel though.

The Last Olympian
Shadow traveling was first shown by Mrs O'Leary, who took Nico and Percy to May Castellan, Luke's mom, at Nico's request to figure out Luke's trail and memories (and to better help Percy fight Luke since it was realized that Luke had taken a bath in the River Styx). Later the hellhounds from Kronos' army shadow traveled inside the defenders borders to attack during The Battle of Manhattan in Manhattan. Nico is also able to shadow travel, but it requires a lot of effort and it leaves him exhausted.

After an attack from the mountain gods, Nico mentions that he could shadow travel past the mountains if he were on his own and his strength was up.
When they reach the House of Hades, they are separated. Frank calls in a favor from the dead to transport the Seven to where Hazel is battling Clytius. It is unknown what type of travel this is, shadow or other. After the battle with Clytius, the ceiling starts to break and crumble. Hazel asks Nico whether he could shadow-transport them, but Nico says he can hardly transport himself. Then, Hazel offers to help and they both successfully transport the entire team to a hillside overlooking the River Acheron. Nico then offers Reyna his help to transport the Athena Partheno to Camp Half-Blood via shadow travel.

Hellhounds

 * Cerberus
 * Mrs. O'Leary
 * Other unknown hellhounds

Demigods

 * Nico di Angelo
 * Hazel Levesque

Gods

 * Hades
 * Persephone

Trivia

 * In other literature, Shadow Travel is sometimes referred to as Umbraportation.
 * Shadow Travel is an ability that seems to be shared by most Underworld monsters and deities, meaning gods like Melinoe may also have this ability, but haven't used it in the series.