While casting a spell without paying its mana cost using the last ability, you can't pay alternative costs, but you may choose to pay any optional additional costs the spell may have (such as a kicker cost)
Paired with Sam, Loyal Attendant, Frodo fits neatly into a lifegain strategy that snowballs value while leaning into the flavorful Ring mechanic
The strategy is effective, but a bit fragile
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The play pattern here is to get Vivi in play, pitch a huge red spell card to cast a free Blazing Shoal and make Vivi enormous, then use Vivi's ability to generate loads of free mana and cast more giant spells, storm off, or simply swing and deal heaps of damage to your opponent with the now embiggened mage
You can then choose any number of modes, and you're allowed to choose the same mode multiple times