Shadow Travel

Shadow Travel is a form of transportation that allows creatures of the Underworld and demigod children of Underworld deities 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. Children of Pluto can also shadow travel.
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. Percy describes how it feels to shadow travel as "going so fast that it feels like your face is peeling off."
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. The more you do it, the more you lose contact with the physical world.
Deities such as Hecate, Persephone, or Hermes can enter or leave the Underworld at will. Hence, their demigod children can probably shadow travel as well, if properly trained.
== Side Effects ==
In The Blood of Olympus, one side effect was shown besides exhaustion and fatigue: overexertion of shadow travel can cause the user's molecular structure to begin to fall apart, gradually turning them into shadows and losing themselves. Due to Nico pushing himself to bring the Athena Parthenos to Camp Half-Blood, he began to lose himself, even entering a "shadow coma". 
== Users ==
=== Gods ===
* Hades/Pluto
* Persephone
* Nyx
* Erebus
* Hecate
* Thanatos (possibly)
* Melinoe (possibly)
* Hermes (possibly)
* Nemesis
=== Demigods ===
* Michael Young
* Melissa Chang
* Other unknown children of Hades/Pluto
* Demigod/legacy children of Underworld deities
=== Hellhounds﻿ ===
* Cerberus
* Other unknown hellhounds and monsters
== Trivia ==
* In other literature, shadow travel is sometimes referred to as Umbraportation.
* Shadow travel is an ability that seems to be shared by most, if not all, Underworld monsters and deities, meaning gods like Thanatos and Melinoe may also have this ability, but haven't used it in the series.