Lightning Axe

Oracle Text

As an additional cost to cast this spell, discard a card or pay 5.

Lightning Axe deals 5 damage to target creature.

Card Rulings

9/25/2006 This has an additional cost with an option. Essentially, the card’s cost to cast is either or and a discard. But its mana cost is always and its converted mana cost is always 1.
4/8/2016 The converted mana cost of Lightning Axe is 1, independent of which additional cost you paid.