A monster skill is a skill that is can only be used by NPCs, regardless of whether the character is friendly or hostile or even human. They include unique effects (e.g. Stun Immunity), skills needed for special circumstances (e.g. Claim Resource), and special versions of player skills (e.g. Charm Animal (White Mantle)). Typically, they are not tied to an attribute or a particular profession. Monster skills cannot be captured or copied (e.g. via Arcane Thievery).
Many monster skills use a common skill icon , but others use their own icons (usually golden in color) or icons similar to that of the associated player-usable skill.
 Passive skill
A number of monster skills are passive, i.e. their effects are always on so the skill is never directly activated (and thus never visibly seen by the player). For example, many Giants cannot be knocked down because they carry Stun Immunity and Undead are especially vulnerable to Holy damage from carrying Undead Sensitivity to Light.