When an opponent's monster declares an attack: Your opponent chooses 1 random card from your hand, then if it is a monster that can be Special Summoned, Special Summon it. Otherwise, send it to the Graveyard.
Rulings
Individual Card FAQs
You can Special Summon a high-level monster, like "Dark Magician", with "A Hero Emerges". But if your opponent selects a Special Summon-only monster like "Dark Necrofear", or a Spirit Monster, it is sent to the Graveyard.
When "Guardian Elma" is selected for "A Hero Emerges", and you have "Butterfly Dagger - Elma" on your side of the field, you can Special Summon her, but if you don't have it, then she is sent to the Graveyard.
You can activate "A Hero Emerges" even if you have only 1 card in your hand.
Netrep Rulings
This ruling addresses what happens if your opponent chooses the following monster card from your hand by the effect of "A Hero Emerges."Special-Summon-only monster: sent to the Graveyard. (Perfectly Ultimate Great Moth, Exodia Necross, Dark Necrofear, Chaos Emperor Dragon - Envoy of the End, all Ritual Monsters, etc.)Spirit Monster: sent to the Graveyard. (Yata-Garasu, Hino-Kagu-Tsuchi, Yamata Dragon, Tsukuyomi, etc.)Guardian Monster without appropriate Spell Card on the field: sent to the Graveyard. (Guardian Grarl, Guardian Ceal, Guardian Tryce, etc.)
Special-Summon-only monster: sent to the Graveyard. (Perfectly Ultimate Great Moth, Exodia Necross, Dark Necrofear, Chaos Emperor Dragon - Envoy of the End, all Ritual Monsters, etc.)
Spirit Monster: sent to the Graveyard. (Yata-Garasu, Hino-Kagu-Tsuchi, Yamata Dragon, Tsukuyomi, etc.)
Guardian Monster without appropriate Spell Card on the field: sent to the Graveyard. (Guardian Grarl, Guardian Ceal, Guardian Tryce, etc.)
Trap|
Normal