REDU format ruling
Black Horn of Heaven

Nearest-era rulings (Tengu, Sept 2011) — no REDU-specific archive exists.
Card effect (current official text)When your opponent would Special Summon exactly 1 monster: Negate the Special Summon, and if you do, destroy it.
Rulings
- Like "Horn of Heaven", this card cannot be used against "Premature Burial", etc. It can be used against inherent Special Summon effects like "Cyber Dragon" or "Dark Necrofear".
- You cannot activate "Black Horn of Heaven" when you Special Summon "Lava Golem" onto your opponent’s side of the field.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the REDU forum — sourced change requests get reviewed.