Grim Reaper's Sprint

The morbid discount reframes the whole card: pay full price and it is an overcosted haste enabler, but let one creature die first and it collapses to two mana, at which point it stops behaving like an aura and becomes an alpha-strike button. The extra combat phase is the payload, and it only fires from a main phase, which locks the sequencing tighter than it looks. Because the untap is an enter-the-battlefield trigger, you do not cast this before combat; you swing first, let creatures trade, then cast it in your postcombat main phase to bank the death for the discount, untap everything that just attacked, and take a second combat with a full, refreshed board crashing into a defense thinned by the first wave. Extra-combat effects usually live on sorceries and instants (Relentless Assault, Aggravated Assault, the Fury of the Horde line), where they buy one team-wide swing and vanish; hanging the effect on an aura instead ties the reward to a single body and asks you to commit that creature, now +2/+2 and hasty, to the attack. The untap clause is what quietly widens the payoff past that one attacker, resetting the whole team rather than the enchanted creature alone. The death-gated discount is the entire loop's viability, so the card wants a sacrifice outlet or an already-bloody board to reach its real rate.