Imps are the lowest type of spirit, and are generally quite rude, thus only performing well as a messenger, or in large numbers of them. Imps are commonly also trapped in reflective surfaces to create scrying glasses. Skrikers are a disreputable sub-type of imps, and Natterjack Implings are good as harmless sub-type.
Like all spirits, imps possess some form of magic, but it is extremely weak compared to the others. Unlike other demons, they are unable to change form and thus gain respite from the pain the Earth's pull causes on their essence. As the lowest type of spirit, they are easily summoned by even mildly-competent magicians in large numbers.