Counter target spell unless its controller discards their hand.
Transmute {1}{U}{B} ({1}{U}{B}, Discard this card: Search your library for a card with the same mana value as this card, reveal it, put it into your hand, then shuffle. Transmute only as a sorcery.)
If the spell’s controller has no cards in their hand, that player can still choose to discard their hand and prevent the spell from being countered.