Disloyal is a role modifier conceived as a complement to the earlier Loyal modifier. If a player is Disloyal, then any attempts by that player to use a night action on a player with the same alignment will fail (as though they had been roleblocked). As such, a disloyal player can only succeed with actions when targeting a player of a different alignment.
Disloyal is an acceptable modifier under Normal guidelines on mafiascum.net, when used on an active role belonging to a player of any Town or Mafia/Werewolf alignment. It causes the role to behave as though it were roleblocked unless the target has a different alignment than the user.
Use and Power
There are two main influences a Disloyal role can have a setup.
One is to avoid combinations of actions that the moderator feels would be overpowered, underpowered, or excessively swingy. If a member of the Mafia has a Disloyal role, then they will know who they can and can't use it on (thus making the ability entirely Negative Utility in this case); for example, a Mafia Disloyal Watcher, which can be used by the Mafia to hunt for town power roles, but cannot alert them to investigations or other actions targeting the Mafia themselves. This kind of "toning down" can happen in the Town too; a Town Disloyal Cop will get no result on investigating Town, the same as if they were roleblocked, so they cannot actually be sure the target is Town.
Alternatively, there are cases in which a role can be made more powerful via the addition of this modifier; a Disloyal Vigilante is incredibly powerful in Town's hands, as it can effectively be used to investigate targets and immediately kill them only if guilty. In general, anyone with a Disloyal role as town should consider whether it's possible to gain investigative-style information from the fact that the role fails on anti-Town players.