The ability that defines Scourge of the Skyclaves's power and toughness applies in all zones, not just the battlefield.

If the highest life total among players is greater than 20, Scourge of the Skyclaves has negative power and toughness equal to the difference.

In a Two-Headed Giant game, Scourge of the Skyclaves causes each team to lose twice half their life rounded up, which is their full life total or, if it's odd, that much life plus 1. This causes the game to end in a draw.

An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered. Notably, Scourge of the Skyclaves may have its toughness change from less than 0 to greater than 0 as a result of its triggered ability before it resolves and enters the battlefield.

If you put a permanent with a kicker ability onto the battlefield without casting it, you can't kick it.

If a card or token enters as a copy of a permanent, the new permanent isn't kicked, even if the original was.

If a spell's kicker cost was paid, the spell is "kicked."

If you copy a kicked spell on the stack, the copy is also kicked. If the copied spell is a permanent spell, the token the copy of that spell becomes when it enters is also kicked.

The kicker ability doesn't let you pay a kicker cost more than once.

To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.