The halo effect refers to the tendency to illogically assign a host of favorable traits to an individual when only one actual favorable trait is known. For instance, if I were to show a test subject a photograph of a physically attractive person, and ask the test subject to rate in another of other areas (e.g., intelligence, social adeptness, wisdom, psychological stability), the test subject - as a result of the halo effect - would rate the person in the photograph as being high on those other traits (i.e., the test subject would opine that the person in the photograph is also intelligent, socially adept, wise, and psychologically stable). That would be, of course, illogical, since the test subject really has no basis for rating that person on the other attributes, but nevertheless inflates them simply as a product of the person's physical attractiveness. Hence, the halo effect.
What I think you may be referring to here is the devil effect, which is based on the same principle, but just opposite in effect of the halo effect - a person is rated as being negative in numerous areas when it is judged that they are negative in one area. So, in the above example, a physically unattractive person in the photograph would be rated lower in other areas (e.g., the would be judged to be less intelligent, socially inept, naive, and psychologically unstable).