Lightning Axe
Lightning Axe SOI
Shadows over Innistrad Uncommon Time Spiral Common 
Cost: Mana R
CMC: 1
Card Type: Instant
Oracle Text: As an additional cost to cast Lightning Axe, discard a card or pay Mana 5.

Lightning Axe deals 5 damage to target creature.

Flavor Text: Time Spiral Common : "A gargoyle's meat can be carved with an ordinary cleaver, but for its petrous hide . . ."
The Underworld Cookbook

Shadows over Innistrad Uncommon : Spirits disturbed by ghoulcallers become geists driven by restless anger.

2006-09-25 This has an additional cost with an option. Essentially, the card’s cost to cast is either Mana 5Mana R or Mana R and a discard. But its mana cost is always Mana R and its converted mana cost is always 1.
2016-04-08 The converted mana cost of Lightning Axe is 1, independent of which additional cost you paid.