Scourge of the Throne rewards attacking the player who is ahead on life. Its first attack each turn can untap your attacking creatures and add another combat phase if that attack is directed at a player with the most life or tied for most life. Dethrone also adds a +1/+1 counter when the appropriate player is attacked, giving the Dragon some growth alongside the extra combat.
The condition creates a real targeting decision. An opponent with the highest life total may have the strongest defenses, while the easiest player to attack may not qualify. Haste from Lightning Greaves or another source helps the six-mana Dragon exploit an opening immediately. The first attack can then prepare your existing attackers for another combat, multiplying useful attack triggers and damage opportunities.
Check life totals carefully when the extra-combat trigger resolves, since its condition matters at that point as well. The ability adds no main phase between combats, and it untaps attacking creatures rather than the entire board. Scourge also does not give haste to creatures that just entered. It is strongest in Dragon or combat decks that can direct a safe attack at the qualifying opponent and retain useful attackers for the added phase. Extra combat is a substantial reward, but planning a viable second attack matters more than simply producing the phase.
