Spend mana

Spend mana is an ability that spends extra mana beyond the actual mana cost of the card to create an additional effect. These are often called Proportional effects because the effect is proportional to the mana spent, although other Hearthstone mechanics are proportional as well.

The "Forbidden" family of cards introduced in Whispers of the Old Gods were the only cards with this ability in the game, until the mechanic was reintroduced with Forbidden Words from Rise of Shadows. Most are spells with a mana cost of 0, and each one consumes all remaining mana when played.

Strategy

 * Players can determine how much mana to leave for the "Spend mana" ability by expending mana in other ways first, leaving a smaller amount remaining for the ability.
 * After playing a "Spend mana" card, you can not play any other cards or use your Hero Power in the same turn, unless you have 0-Mana cards (e.g. Moonfire, Target Dummy), or cards that allow you to regain Mana (e.g. The Coin or Innervate). This makes it wise to carefully assess the desired sequence of events, make use of your Hero Power and possibly play other cards, before finally playing the "Spend mana" card.

Trivia

 * Spend mana currently represents the only ability or effect that consumes the player's available mana. Mana is of course naturally consumed to pay the base cost of a card or Hero Power. Some cards also destroy Mana Crystals, although these almost always destroy empty Crystals without affecting available mana.

Patch changes

 * Added, with Forbidden Shaping, Forbidden Healing, Forbidden Ancient, Forbidden Ritual and Forbidden Flame.