Aetherflux Reservoir (Retro Schematic) [The Brothers' War Retro Artifacts] Nahiri Set: Unknown Event Type: Creature

Set: Unknown Event Type: Creature — Phyrexian Imp Rarity: Common Cost: {4}{B/P} (B/P can be paid with either B or 2 life

they put a card they own from outside the game into your hand

Set: Double Masters Type: Land — Urza's Mine Rarity: Rare {T}: Add {C}

Shatterskull Pass is a pretty distant second

While it's phased out

Aetherflux Reservoir (Retro Schematic) [The Brothers' War Retro Artifacts] Nahiri Set: Unknown Event Type: CreatureSet: The Brothers' War Retro Artifacts Type: Artifact Rarity: Mythic Cost: {4} Whenever you cast a spell, you gain 1 life for each spell you've cast this turn. Pay 50 life: Aetherflux Reservoir deals 50 damage to any target. "These estimates are conservative. Its true capacity may be far, far greater." Research notes, signature illegible