(1) Once per turn, when a Spell Card is activated during your opponent's Main Phase, you can Special Summon 1 Level 4 DARK monster from your GY.
The (1) optional Trigger Effect activates AFTER a Spell Card's activation resolved (starting a new chain).
This effect CAN miss the timing if Chain Link 1 was NOT the activation of a Spell Card.
This effect WILL NOT trigger in response to the effect of face-up Spell Card (ex: Black Garden).
This effect WILL NOT trigger if Dark Horus is flipped face-down before it can activate its effect.
This effect WILL NOT resolve if Dark Horus is flipped face-down before resolving its effect.
The effect of "Dark Horus" starts a new Chain after the Chain including the Spell Card resolves.
If an effect is chained to the effect of "Dark Horus" that flips "Dark Horus" face-down or removes it from the field, its effect disappears.