If the spell you cast that caused Kellan, the Kid’s last ability to trigger has {X} in its mana cost, X is the value chosen as that spell was cast for the purpose of determining its mana value.

If the spell you cast that caused Kellan, the Kid’s last ability to trigger is represented by a split card, the characteristics of the half you didn’t cast are ignored while that spell is on the stack. For example, if you cast the Cease half of Cease // Desist from the Murders at Karlov Manor release, that spell’s mana value is 2 because Cease’s mana cost is {1}{B}{G}.

A permanent spell is an artifact, battle, creature, enchantment, or planeswalker spell.

If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, those must be paid to cast the spell.

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

Putting a land card onto the battlefield with Kellan’s last ability doesn’t count as playing a land. It can put a land card onto the battlefield even if you’ve already played your land for the turn.