Mana Burn will destroy 10% of the target's mana. If the actual amount is greater than 20% of your maximum mana, it will destroy only an amount equal to 20% of your maximum mana. This is necessary to prevent Mana Burn from destroying too much mana on enemies with a very large mana pool.
Mana Burn will instantly break Fear, Hex and Psychic Scream.
Mana Burn is useful both in PvE and PvP. It can be used against enemy spell casters in PvE to prevent them from casting their spells, especially healing spells. In PvP this spell is helpful against healing classes.
With Improved Mana Burn, its effectiveness can be raised enormously. In addition, its susceptibility to interrupt effects can be reduced.
Combined with a shadow spec'd priest's Vampiric Embrace, Mana Burn can be an amusing and effective way to use the target's mana to heal your party (while damaging them in the process).
due to their low mana (no intellect stacking) this can quickly make a retribution or protection paladin run out of mana very quickly, if the priest can survive the innitial enslaught of damage, turning the dreaded paladin into a laughing stock for priests everywhere