The musical sound of its turning triggers the tumblers of revelation.

Cloud Key

As Cloud Key enters, choose artifact, creature, enchantment, instant, or sorcery.
Spells you cast of the chosen type cost {1} less to cast.

The Brothers' War Retro Artifacts

As Cloud Key enters the battlefield, choose artifact, creature, enchantment, instant, or sorcery.
Spells you cast of the chosen type cost {1} less to cast.

"A key only unlocks a door. Its wielder must choose whether to cross the threshold."
—*Hurkyl's Metaphysics, vol. II*

The List

Time Spiral Remastered

The cost reduction applies only to generic mana in the cost of spells of the chosen type you cast.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of Cloud Key's ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.