Horn Drill is a One-Hit KO move (a.k.a. OHKO).
OHKOs deal enough damage to the opposing pokemon to knock it out when the move hits. This is an important distinction from a move that simply causes the opponent to faint (like Perish Song). As a result of this, OHKOs are subject to type immunities and the effects of Endure and the item Focus Band. OHKOs cannot affect an opposing pokemon that is of a higher level than the pokemon using the move. Additionally, a bonus is added to the base accuracy of the move if the using pokemon is of a higher level than the opponent. The bonus is equal to ((Attacker's level - Target's level)/256).
If it was not made explicitly clear enough in the pervious paragraph, since Horn Drill deals damage, a Ghost-type pokemon's immunity to Normal attacks will protect it from Horn Drill.
OHKOs include: Fissure, Guillotine, and Horn Drill
Horn Drill is affected by Brightpowder when the opposing pokémon is equipped with it.