Sleep

The clean version of an effect aggressive blue decks have always wanted but rarely earned: not a fog, not a tempo bounce, but a full forced lock of an opponent's board across two combat steps. The "don't untap" clause does the heavy lifting. Tapping every creature buys one swing; denying the next untap step turns the spell into a near-certain second swing on the following turn, when those creatures are still sideways and the controller can only block with whatever they can freshly cast. That is the structural trick: it converts a one-time tempo gain into two consecutive largely unopposed attacks, which makes it a finisher in a deck that can capitalize on the window rather than a stall in one that cannot. The cost is the honest part of the design. Four mana at sorcery speed means it cannot answer creatures already attacking and nothing at instant speed, so it cannot snare an attacking force the way a true Fog can; it has to be cast proactively, on your own turn, as a setup for lethal. That sorcery restriction is what buys the right to print a "tap everything" effect this absolute at this rate: it can only ever press an advantage you already hold, never reverse one you have lost.