Heavy Fog

The casting restriction is the entire design: this fires only during the declare attackers step, and only once an attack has actually come at you. That is a tighter window than ordinary Fog, which is also an instant but carries no condition: you can hold up Fog and cast it whenever you like, including on turns nobody swings or in response to a noncombat effect. Heavy Fog cannot. It is locked to the precise moment creatures turn sideways at you, and it does nothing if the moment never arrives. The trade you get for that lost flexibility is a sharpened scope. Where Fog blanks all combat damage, this prevents only the damage that attacking creatures would deal to you. That phrasing is exact and worth reading twice: it protects your life total, not your board. Your blocking creatures still take their full hit and still die in combat. So Heavy Fog is not a way to absorb an alpha strike for free while your blockers trade up; it is a way to take zero life loss while declining to block at all, or while letting unfavorable blocks resolve on the attacker's side without saving the chump. The structure is a timing-locked instant in miniature: take a known prevention effect, bolt the casting window to a single step under a single condition, and narrow the shield to the player rather than the battlefield.