Suddenly, all the haggling didn't seem so important.

Siege Wurm

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Trample

Ravnica Remastered

The List

Guilds of Ravnica

Magic 2015

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Trample (If this creature would assign enough damage to its blockers to destroy them, you may have it assign the rest of its damage to defending player or planeswalker.)

The Rubblefield was once among the wealthiest districts in Ravnica. All it took was a single wurm to reduce it to ruins.

Ravnica: City of Guilds

Convoke (Each creature you tap while playing this spell reduces its cost by {1} or by one mana of that creature's color.)
Trample

Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.

If a creature you control has a mana ability with {T} in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.

Tapping a multicolored creature using convoke will pay for {1} or one mana of your choice of any of that creature's colors.

Tapping an untapped creature that's attacking or blocking to convoke a spell won't cause that creature to stop attacking or blocking.

When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.

You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.