Klauth, Unrivaled Ancient
Most explosive-mana engines decouple the ramp from the swing: they fire on cast, or on a tap ability, letting you generate the flood in any main phase you like. Here the payout is bolted to the attack step, and it scales with the total power of everyone crashing in, so ramp and aggression collapse into a single decision rather than two. A wide field of dragons and beaters turns one attack into a mana pool that survives the usual phase-boundary cleanup, letting you bank it past declare-blockers and empty it into your second main phase. The spend-only clause steers the design away from a machine-gun of activated abilities: this mana casts spells and nothing else, which pushes the reward toward a stormy chain of haymakers instead of an engine that recycles itself through ability costs. Flying and haste do quiet structural work, since the attack trigger wants to fire as fast as possible, and haste means the ancient produces its own opening salvo without ceding a turn to summoning sickness. This is a Gruul "cast everything at once" payoff of a specific stripe: where the archetype usually rewards raw ramp deployed before combat, this one wants creatures already declared as attackers, folding the mana burst into the swing it was going to make anyway.
