Tumbleweed Rising

The value of X is determined only once, as Tumbleweed Rising resolves. If you don’t control any creatures at that time, you’ll create a 0/0 Elemental token, which will likely immediately die unless something else is raising its toughness.

Plot abilities are written “Plot [cost],” which means “Any time you have priority during your main phase while the stack is empty, you may pay [cost] and exile this card from your hand. It becomes plotted.”

If you’re casting a plotted card from exile without paying its mana cost, you can’t choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the plotted card has any mandatory additional costs, those must still be paid to cast the spell.