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 has some strange behaviour in combination with the Shadow tree. It works with Shadow Weaving, Shadowform and Vampiric Embrace. However, it does not benefit from Darkness, Shadow Focus or Shadow Reach. Shadow Affinity probably does not affect it as well.
Its threat simply depends on its damage and follows normal rules; the mana drain does not cause any additional threat.
Improved Mana Burn reduces the casting time of Mana Burn.
Force of Will increases spell damage and critical strike chance for all offensive spells.
Tips and tactics
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
Hotfix (2009-04-30): "Mana Burn has been reduced to 10% mana burned (up to a maximum of 20% of the Priests mana), down from 13% (max 26%). "