Herald's Horn [Commander 2017] Hidden Arsenal Set: Dark Revelation Volume 4

Set: Dark Revelation Volume 4 Card type: Effect Monster Rarity: Secret Rare Attack: 1800 Defense: 1900 Activate this effect only during your Battle Step

but it don't say where to find it

by Tributing the equipped monster and this card

Set: Structure Deck: Lair of Darkness Card type: Effect Monster Rarity: Common Attack: 2800 Defense: 1000 You can Tribute Summon this card by Tributing 1 Tribute Summoned monster

Set: Fusion Enforcers Card type: Normal Spell Rarity: Super Rare Fusion Summon 1 "Frightfur" Fusion Monster from your Extra Deck

Herald's Horn [Commander 2017] Hidden Arsenal Set: Dark Revelation Volume 4Set: Commander 2017 Type: Artifact Rarity: Uncommon Cost: {3} As Herald's Horn enters the battlefield, choose a creature type. Creature spells you cast of the chosen type cost {1} less to cast. At the beginning of your upkeep, look at the top card of your library. If it's a creature card of the chosen type, you may reveal it and put it into your hand.