just did a slightly similar thing but I feel like the best option here is make an expensive (CMC 5+) fog variant. so let's go with that. fog is, by its nature, a cheap effect, so trying to fit it onto an expensive card seems like a fun challenge.

gonna start grading on Friday. go!

Are cost reduction abilities against the spirit of the criteria?

Fog of War
Instant
Prevent all combat damage that would be dealt to you or permanents you control this turn.
Untap each creature you control. If you cast ~ during your main phase, after this phase, there is an additional combat phase followed by additional main phase.

Energy Conversion

Enchantment
Flash
When Energy Conversion enters the battlefield, prevent all combat damage that would be dealt this turn.
Whenever damage is prevented, put that many charge counters on Energy Conversion.
, Remove X charge counters from Energy Conversion: Energy Conversion deals X damage to any target. You gain X life.

Inkshield is a 5-mana fog and sort of the target I'm going for here.

hadn't considered that, but I'm gonna say go for it. as long as its CMC is 5 or more I'll allow it.

Prophesied Return
Instant (R)
Return target creature card from your graveyard to the battlefield.
Prevent all combat damage that would be dealt this turn.
Silence descended on the battlefield. Some in reverence, some in disbelief.

Sudden Forest
Instant
Search your library for up to two basic forests and put them onto the battlefield.
Prevent all combat damage that would be dealt this turn.

Gorillas in the Mist
Instant
Create two 3/3 green Ape creature tokens with trample.
Prevent all damage that would be dealt to creatures this turn.

Misplaced in time
Instant
Prevent all combat damage creatures would deal this turn, then destroy all nonland, noncreature permanents.

Asura Intervention
Instant (R)
Whenever a source would deal damage this turn, prevent that damage, gain that much life, and draw a card.
As the decree to suspend the battle echoed across the sikes, the flying warmachines of Esper and the wyrms of Jund alike descended to sit down in quiet contemplation.

Ominous Distraction
Instant (U)
Cast this spell only during combat.
Prevent all combat damage creatures would deal this turn. At the beginning of the next end step, you may put a creature card with mana value X or less from your hand onto the battlefield, where X is the amount of combat damage prevented this turn.
A legion seen is never as great as an enemy unknown.

Let's just make regular fogs 5 cmc and cut off turbo fog at the source.

Tides of Conflict |
Instant

Fill in the blanks — Distribute [prevent] and [double] among the blank spaces.
________ all combat damage that would be dealt to creatures this turn.
________ all combat damage that would be dealt to players this turn.

Displacement
Instant
Creatures phase out.

I am assuming fog variants means damage prevention specifically.

Words of Reason -
Instant
~ costs less to cast if two or less creatures are attacking you.
Prevent all damage that would be dealt until your next turn. Then, each player scry X, where X is the mana spent to cast ~.
Draw a card.
Peaceful dialogues are simply the prelude to war.

No Time to Die
Instant
Cast No Time to Die only during combat, after blockers are declared.
Prevent all combat damage that would be dealt this turn, and untap all attacking and blocking creatures.
Take an extra turn after this one. During that turn, each creature untapped with No Time to Die must attack or block if able, and no other creatures may attack or block.

Grand Reversal —
Instant (R)
Prevent all combat damage that would be dealt this turn. For each point of damage prevented to you, you gain 1 life. For each point of damage prevented to a creature you control, put a +1/+1 counter on that creature. For each point of damage prevented to a planeswalker you control, put a loyalty counter on that planeswalker.

Inquisitor Vosch
Legendary Creature - Human Cleric (R)
Flash
When Inquisitor Vosch enters, target opponent exiles each nontoken creature they control in a face-down pile, shuffles that pile, then manifests those cards.
"Sinners retreat like rats scurrying from the light. So be it—the Church sees even where the sun does not."
4/4