Excerpted from Marge Hollander's Digest of Demons and Devils:
"The Syl'udet are a common race of parasitic demons. Within the confines of Hell they are as prolific as rats in urban sewers. Most humans will have contact with these dread pests at least once in their lifetimes.
Within the earth plane, the Syl'udet generally cannot be seen by the mundane eye. They are visible briefly during twilight times and then only by those who are sensitive or psychic. However, there are many occult devices that do allow a mundane viewer to perceive these creatures. In two separate recorded incidents, Syl'udet have been perceived using Sheridan's Visor as well as a Veil of the Mudra.
"The Syl'udet are a common race of parasitic demons. Within the confines of Hell they are as prolific as rats in urban sewers. Most humans will have contact with these dread pests at least once in their lifetimes.
Within the earth plane, the Syl'udet generally cannot be seen by the mundane eye. They are visible briefly during twilight times and then only by those who are sensitive or psychic. However, there are many occult devices that do allow a mundane viewer to perceive these creatures. In two separate recorded incidents, Syl'udet have been perceived using Sheridan's Visor as well as a Veil of the Mudra.