Black Horn of Heaven
Negate the Special Summon of 1 of your opponent's monsters, and destroy it.
Very simple, but very powerful. This card works a lot like Thunder King Rai-Oh and Steelswarm Roach as it won't negate a Special Summon that is the result of a Spell, Trap, or Monster Effect. What it does take care of is Synchro Summons, Xyz Summons, and boss monsters.
Solemn Warning is a pretty amazing card because it can stop ANY summon, whether it's from a card effect or not. Black Horn of Heaven lets you save your Warning (and 2000 Life Points) for something like Gorz the Emissary of Darkness or a game-winning Monster Reborn.
You might say that Bottomless Trap Hole is better since it can stop Normal Summons as well and banishes the monster, but with the TCG's priority ruling, that Black Luster Soldier - Envoy of the Beginning or Master Hyperion can still get their effects off with Bottomless Trap Hole. Since Black Horn negates the summon entirely, that means your opponent won't be able to Special Summon that BLS, Synchro Monster, or Xyz Monster from the graveyard.