"Shadow" is the term used to describe the creatures that dwell within the parallel Shadow Universe. The creatures used to rarely emerge, as they cannot survive on Lore. Due to the efforts of the SeekRat, however, a large portion of Lore, known as the Veil, has been permanently corrupted by the Shadow Universe, allowing Shadows to move about freely and corrupting normal creatures that enter.
Many Shadows have taken the forms of creatures found on Lore, such as the Frogzard, the Behemoth, and the Sphinx. Furthermore, a powerful Shadow named Ultimon was able to corrupt many "fake shadows" such as the Shadow Wolf and the Night Terror and turn them into "real" Shadows. Furthermore, thanks to Ultimon's meddling, these Shadows can now survive beyond the Veil, prompting Lord Barrius and the mysterious order he serves to construct a massive wall to hold the creatures within, as they are extraordinarily powerful and would wreak untold havoc across Lore if set free.
That being said, not all Shadows seem to hate life on Lore. The Shadow known as EIBhe, for example, is tolerant of life on Lore and has even sparked a friendship with the Chosen.
All Shadows were created by an ultimate Shadow known to them as Mother. Mother is best described as a force of creation and chaos. Normally slumbering deep within the Shadow Universe, Mother can be awoken and driven to terrible wrath if certain unspecified conditions are met. All Shadows fear her, and she is described as unstoppable even by Lore's deities, as she created their Shadow counterparts.
Most Shadows possess standard resistances to all damage types, excepting light and darkness (which they only take half damage from.) Additionally, they deal light and darkness damage with their attacks, which otherwise function identically to the attacks as their non-shadow counterparts.
However, exceptions exist. The most notable example would be Ultimon, whose resistances are increased by 50% (he takes no damage from light and darkness, and 1/2 from all others) and who can inflict damage onto his opponent equal to the damage he takes in battle. Another exception is the Shadow Sphinx that Lord Barrius can conjure from the Training Grounds. Its whirlwind attack will strip you of your weapons and force your to literally fight bare-handed against the thing.