Arena of Glory

Haste has always been the cheapest keyword to hand out and one of the hardest to bake into a manabase without warping the land into something strange. Red decks have historically leaned on creature-lands and one-off enablers to give a threat its first swing, but those bodies cost cards and slots; here the same job comes from a source that also just produces mana on the turns it isn't doing anything special. Exertion is the lever. The plain tap makes a single red like any Mountain-in-disguise, but the double-red mode skips the next untap, so the tempo you buy is charged against your following turn. That toll is what keeps the acceleration from being free: instead of a card, you spend a future turn's mana to make a haymaker connect early. The enters-tapped clause anchors it to a genuine red base rather than a light splash, and because the exert cost itself demands a red pip, the ability wants you already flush before it fires rather than scrambling to enable it. The window it serves is precise: a red deck has drawn its finisher and would rather swing now than pass and wait, and this charges a measured price for the privilege instead of pretending speed comes without cost.