Imminent Doom enters the battlefield with a doom counter on it.
Whenever you cast a spell with converted mana cost equal to the number of doom counters on Imminent Doom, Imminent Doom deals that much damage to any target. Then put a doom counter on Imminent Doom.
7/14/2017 Imminent Doom’s triggered ability resolves before the spell that caused it to trigger. The ability will resolve even if that spell is countered.
7/14/2017 If a spell has in its mana cost, include the value chosen for that X when determining the converted mana cost of that spell.
7/14/2017 The amount of damage Imminent Doom’s triggered ability deals is the number of counters it had on it as the ability triggered. For example, if you cast Shock and then respond to Imminent Doom’s triggered ability with a second Shock, both abilities cause Imminent Doom to deal 1 damage and Imminent Doom will end up with three doom counters on it.