Kellan Joins Up

The mana value of a split card in your hand is based on the combined mana cost of its two halves.

If a card doesn't have a mana cost, its mana value is 0.

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.

If a plotted card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If multiple legendary creatures enter the battlefield under your control at the same time, Kellan Joins Up's last ability will trigger once for each of those creatures.

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."