Shadow Travel

Shadow travel is a form of transportation that allows creatures of the Underworld to travel to any desired place on Earth or the Underworld, although it makes the user extremely tired.

Shadow Traveling
Hellhounds and children of Hades use this traveling method by slipping into the shadows and coming out somewhere else. 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 for Mrs. O'Leary. She was still running after they got into the shadows when Percy and Nico were traveling to Westport, Connecticut to find May Castellan. Shadow traveling exhausts the user badly; for example, Nico said that he passed out for a week when he first tried it, and Mrs. O'Leary herself was tired after doing it. Percy describes the feeling of Shadow Traveling as going so fast that it feels like your face is peeling off.

The Demigod Files
In the short story Percy Jackson and 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. This was not mentioned as being shadow travel, though.

The Last Olympian
Shadow traveling was first shown in The Last Olympian by Mrs O' Leary, who took Nico and Percy to May Castellan, Luke's mortal 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 leaves him exhausted.

Hellhounds
Orthud
 * Cerberus (likely)
 * Mrs. O'Leary
 * Other unknown hellhounds

Children of Hades

 * Nico di Angelo

Others
Hecate
 * Hades
 * Persephone (likely)
 * Melinoe (likely)

Trivia

 * In other literature, Shadow Travel is sometimes referred to as Umbraportation.