“Horus’ Servant” has an effect just like “Lord of D.”, but only applicable to “Horus the Black Flame Dragon”. “Horus’ Servant” will not negate a card effect. All it does is prevent “Horus the Black Flame Dragon” from being selected as a target. If your opponent selects “Horus the Black Flame Dragon” as a target (with “Ring of Destruction”, for example), and you chain “Call of the Haunted” to Special Summon “Horus’ Servant”, then “Horus the Black Flame Dragon” will still be destroyed by “Ring of Destruction” because “Horus’ Servant” is being Special Summoned AFTER “Horus the Black Flame Dragon” was selected. The fact that “Ring of Destruction” has not yet resolved does not matter; “Horus the Black Flame Dragon” was selected at activation of “Ring of Destruction”, and “Horus’ Servant” is Special Summoned too late to prevent this.
Your opponent cannot activate a Spell Card, Trap Card, or Monster Effect that would select “Horus the Black Flame Dragon” as a target while “Horus’ Servant” is in play. “Horus’ Servant” does not cause the effect to be negated; he prevents the activation of the effect in the first place.