The release of Duelist Legacy 4 changed some wordings on cards -- one of them being Ascending Horn (e.
Horn of Heaven), and also relates to Declaration of God (e. Solemn Judgement).
The rulings of the card with the new wording (and this includes Declaration of God) is that you can now ONLY negate Special Summons that are brought out via their own, inherent effects.
What does this mean? Cards that use their own inherent effects to Special Summons them include the
Gate Guardian, Dark Necrophia (e. Necrofear), the Necrophia cousins from
Labyrinth of Nightmare, and even Giga Cyber (e. Megacyber). THESE are the cards that Horn/Declaration can negate.