Protects your Horus monster from all card effects.
Very low stats make him very easy to get rid of.
Horus' Servant does not negate effects. Therefore if your opponent has already selected Horus the Black Flame Dragon as the target for an effect, and then you special summon Horus' Servant, the effect will still go through.
Horus the Black Flame Dragon
Here are the monsters you can protect with Horus' Servant' effect.