Mana burn

From MTG Salvation Wiki

Mana burn used to be loss of life for not using mana in your mana pool when a phase ends. For instance, you tap a Swamp to cast Dark Ritual which gives you which you use to cast Erg Raiders (which cost ). If you can't use the remaining then you will lose 1 life from mana burn.

The rules update to Magic 2010 has removed this part from the game. Players do not suffer loss of life for unused mana any more.

THIS RULES TEXT IS OBSOLETE From the Comprehensive Rules: 300. General 300.3. When a phase ends (but not a step), any unused mana left in a player’s mana pool is lost. That player loses 1 life for each one mana lost this way. This is called mana burn. Mana burn is loss of life, not damage, so it can’t be prevented or altered by effects that affect damage. This game action doesn’t use the stack. (See rule 406, “Mana Abilities.”)

[edit] In-world meaning

In storylines mana burn happens when a wizard holds mana and doesn't use it to cast the spell. Such a wizard can seriously suffer from it, be killed, or even be burned to ash if the amount of mana was too big. Some victims of mana burn can be also turned to the Fallen.