Kheru Spellsnatcher

Morph {4}{U}{U} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)
When this creature is turned face up, counter target spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard. You may cast that card without paying its mana cost for as long as it remains exiled.

Duskmourn: House of Horror Commander

Morph {4}{U}{U} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)
When Kheru Spellsnatcher is turned face up, counter target spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard. You may cast that card without paying its mana cost for as long as it remains exiled.

Khans of Tarkir

Commander 2019

A permanent that turns face up or face down changes characteristics but is otherwise the same permanent. Spells and abilities that were targeting that permanent, as well as Auras and Equipment that were attached to the permanent, aren't affected.

Any time you have priority, you may turn the face-down creature face up by revealing what its morph cost is and paying that cost. This is a special action. It doesn't use the stack and can't be responded to. Only a face-down permanent can be turned face up this way; a face-down spell cannot.

At any time, you can look at a face-down spell or permanent you control. You can't look at face-down spells or permanents you don't control unless an effect instructs you to do so.

Because the permanent is on the battlefield both before and after it's turned face up, turning a permanent face up doesn't cause any enters-the-battlefield abilities to trigger.

Because you're already casting the card using an alternative cost (by casting it without paying its mana cost), you can't pay any other alternative costs for the card, including casting it face down using the morph ability. You can pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.

If a face-down permanent leaves the battlefield, you must reveal it. You must also reveal all face-down spells and permanents you control if you leave the game or if the game ends.

If the card has {X} in its mana cost, you must choose 0 as the value for X when casting it.

Morph lets you cast a card face down by paying {3}, and lets you turn the face-down permanent face up any time you have priority by paying its morph cost.

The face-down spell has no mana cost and has a mana value of 0. When you cast a face-down spell, put it on the stack face down so no other player knows what it is, and pay {3}. This is an alternative cost.

When the spell resolves, it enters the battlefield as a 2/2 creature with no name, mana cost, creature types, or abilities. It's colorless and has a mana value of 0. Other effects that apply to the creature can still grant it any of these characteristics.

You can target a spell you control with Kheru Spellsnatcher's triggered ability. This will give you the ability to cast the card later without paying its mana cost.

You must ensure that your face-down spells and permanents can easily be differentiated from each other. You're not allowed to mix up the cards that represent them on the battlefield in order to confuse other players. The order they entered the battlefield should remain clear. Common methods for doing this include using markers or dice, or simply placing them in order on the battlefield.

Morph

(You can cover a face-down creature with this reminder card.
A card with morph can be turned face up any time for its morph cost.)