Immortal Phoenix

unit

[Assault 2] (+2 might while I'm an attacker.) When you kill a unit with a spell, you may pay energy 1rune fury to play me from your trash.

Rulings

• What happens when I play Hextech Ray on an Elder Dragon while Imperial Decree’s delayed trigger is active and I have an Immortal Phoenix in the Trash? Can I play Immortal Phoenix from the Trash?
• What does it mean to “kill a unit with a spell?” It seems like Hidden Blade counts, but what about a spell that deals damage? A spell that causes a unit to deal damage? A spell that causes a unit to die in some other way, like Stupefy on a unit that has 1 more Might than it has damage currently marked on it?
• Can Immortal Phoenix return if it is the unit that was killed? Or if it and other units were killed at the same time? For example, can I Hidden Blade on my own Phoenix and immediately replay it?