If the activation of "Great Horn of Heaven" is negated, the effect is not applied. ("Negate the Special Summon, and if you do, destroy that monster(s), then your opponent draws 1 card, then end the Main Phase." is not applied.)
"Great Horn of Heaven" can be activated when the opponent Pendulum Summons 2 or more monsters. In this case, all monsters that would be Pendulum Summoned are destroyed, but even if 2 or more monsters are destroyed, your opponent only draws 1 card.