You can choose any value for X. Choosing a higher value for X lets you pay more mana and thus spend more colors of mana to cast it. For example, if you choose 0 for the value of X and pay {U}{U} to cast Sweep the Skies, you'll create one Thopter. If you choose 4 for the value of X and pay {W}{U}{U}{B}{R}{G}, you'll create five Thopters.
2021-06-18
wotc
Unless a spell or ability allows you to, you can't choose to pay more mana for a spell with a converge ability just to spend more colors of mana. Likewise, if a spell or ability reduces the amount of mana it costs you to cast a spell with converge, you can't ignore that cost reduction in order to spend more colors of mana.
2021-06-18
wotc
The maximum number of colors of mana you can spend to cast a spell is five. Colorless is not a color. Note that the cost of a spell with converge may limit how many colors of mana you can spend.
2021-06-18
wotc
If you cast a spell with converge without spending any mana to cast it (perhaps because an effect allowed you to cast it without paying its mana cost), then the number of colors spent to cast it will be zero.
2021-06-18
wotc
If there are any alternative or additional costs to cast a spell with a converge ability, the colors of mana spent to pay those costs will count.
2021-06-18
wotc
If a spell with a converge ability is copied, no mana was spent to cast the copy, so the number of colors of mana spent to cast the spell will be zero. The number of colors spent to cast the original spell is not copied.
Format Legality
Standard
Not Legal
Future
Not Legal
Historic
Not Legal
Timeless
Not Legal
Gladiator
Not Legal
Pioneer
Not Legal
Modern
Legal
Legacy
Legal
Pauper
Not Legal
Vintage
Legal
Penny
Legal
Commander
Legal
Oathbreaker
Legal
Standardbrawl
Not Legal
Brawl
Not Legal
Alchemy
Not Legal
Paupercommander
Not Legal
Duel
Legal
Oldschool
Not Legal
Premodern
Not Legal
Predh
Not Legal
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