|The Sleep of Reason Produces Monsters|
(Francisco de Goya, c.1797)
Lurkers are invisible beings and only can be detected by magical or other appropriate supernatural methods. Lurkers are immune to all physical attacks and magical attacks and can only be manipulated to leave the town (charm or other similar).
Even though lurkers don't have physical form for xp they are HD7 monsters. They save as lvl 7 wizards. Lurkers doesn't have physical attacks and if they are found they try to flee and hide.