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 very large mana pools.
Its threat simply depends on its damage and follows normal rules; the mana drain does not cause any additional threat.
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.
Mana Burn is a powerful and often controversial spell in PvP.
Mana Burn is primarily useful for running a healer out of mana, allowing either the healer or an ally to be defeated.
Mana Burn is far less useful against DPS specs; most have quite efficient ways of regenerating mana. For Holy and Discipline priests, using Mana Burn on a non-healer is often a waste of time and mana.
Be aware that an oom healer will often use mana regeneration effects, or try to drink. Great care should be taken to prevent drinking, and any mana regeneration buffs ([Divine Plea], [Innervate]) should be dispelled immediately.
When anticipating the target's mana regeneration, remember to expect effects such as [Rapture]. Priests will use [Shadowfiend] to restore mana, and saving [Psychic Scream] for this purpose can be very effective in preventing this form of regeneration.
Remember that Mana Burn costs you mana. When trying to keep an opponent continually out of mana, the priest can very easily burn himself dry as well. Mana Burn has a fixed mana cost, which will be used in full regardless of how much mana the target loses. Since healers can cast at least one heal with very little mana, it is very difficult to keep targets entirely oom. Priests should only risk burning themselves dry if the healer's teammate is very close to defeat.
One of the main difficulties in using Mana Burn in PvP is its long cast time. Draining a target's mana requires multiple casts, preventing the priest from healing, attacking, moving or performing any other functions while he does so.
Line of sight is also a prime problem in using Mana Burn in PvP. Targets must be in range and in the priest's line of sight at the beginning and end of the cast. This can be difficult to achieve with pillars and obstacles to hide behind. Using teamwork to lead your opponents away from such obstacles can be useful for luring the healer out from safety.
Stuns, snares and other forms of crowd control can be very useful for allowing a quick series of Mana Burns on a healer. [Psychic Scream] can be very useful for this, but be aware that the fear effect will break immediately upon a successful Mana Burn. However, it is sometimes possible to complete a second cast before the target can run behind cover.
Haste effects can be used immediately before a series of Mana Burns, to great effect.
As part of the Shadow School, it is generally more problematic for Shadow priests to be interrupted while casting Mana Burn. Discipline and Holy priests can simply return to using heals and holy spells while the school is locked. One advanced tactic when being attacked by melee classes is to cast Mana Burn in order to force them to use their interrupts; once they have done so, the priest is free to cast healing spells for several seconds without fear of interruption.
Shape-shifted druids cannot be Mana Burned (as their mana bar is replaced with another resource), and Mana Burn cannot be cast on them while they are shape-shifted. With a 2.5-second cast, this makes it extremely hard to Mana Burn druids, who can easily pop in and out of other forms while keeping their target healed. One possible tactic is to use crowd control abilities such as stuns while the druid is in caster form, giving you a chance to burn some of their mana.
While Mana Burn can be a critical tool for priests in PvP, be careful not to become over-focused on burning your opponent. Healing priests especially need to keep half an eye on their teammates, as the loss of a partner is almost always more important than Mana Burning an opponent.
Defence against Mana Burn
Many players complain that Mana Burn is overpowered in PvP, especially in 2vs2 arena. In practice, this is usually mostly due to a lack of teamwork and coordination, and a lack of knowledge and tactics regarding Mana Burn. Incorporating a few of these techniques into your play will quickly make Mana Burning a far more difficult task for the enemy priest to achieve.
Probably the best defence against Mana Burn is simply to move out of line of sight. If a priest is Mana Burning you, find a pillar or obstacle and get behind it. Moving out of range also prevents Mana Burn.
Because you only need to be behind cover when the cast completes, it is quite possible to pop out for a couple of seconds to heal your teammate, before ducking back under cover. Keep an eye on the priest's raid frame. If you see him starting to cast Mana Burn, get behind cover. Meanwhile, try not to get caught out in the open when facing a Mana Burning priest, unless you are confident that you can interrupt him in time. There is usually little reason to be more than a couple of seconds from something you can hide behind. While it can be challenging with kiting classes, encourage your teammate to stay within range as much as possible.
Interrupts are also excellent counters to Mana Burn. All classes except priests and hunters have one, and if Mana Burn is a problem, these can be saved for interrupting it.
Stuns, silences and other effects can also be used to interrupt Mana Burn casts. Again, if Mana Burn is a key problem, try saving these abilities for when the priest begins casting it.
Pushback from damage will also slow Mana Burn casts by up to 1 second.
Players should not forget to use teamwork to counter Mana Burn. Asking your partner/s to save their interrupts for Mana Burn will quickly make it impossible for the priest to get a single cast off. Other crowd control abilities can serve the same purpose, so players need not save all their cooldowns just for this.
Keep an eye on the priest's raid frame. This allows you to anticipate their casts, duck behind cover, interrupt or CC them. Encourage your teammates to do the same.
Remember to make the most of your mana regeneration abilities. Although a poor second to avoiding Mana Burn, healers who find their mana stores rapidly diminishing should always maximise their regeneration. Remember that priests use their own mana by Mana Burning you, so by regularly restoring your own mana it is possible for the priest to burn themselves dry in their efforts to burn you. If possible, choose opportune moments to drink. A few seconds of drinking can rapidly turn the tables on a Mana Burning priest, and the possibility that you might try to drink is often enough to cause the priest to follow you out of range of his own target.
By combining sensible positioning (line of sight), use of interrupts and crowd control abilities, and a little teamwork, healers need not fear Mana Burn.
On occasion, priests may find themselves facing each other in a "Burn Off". In this situation, time is of the essence. The winner of the match is often - though not always - decided by which priest is able to burn the other first. Instructing your partner to help as much as possible can be useful, as can use of all available haste effects - once you're sure you won't be interrupted. However, be careful you don't get so focused on Mana Burning the other priest that you allow your partner to be killed.
When Mana Burning each other, Shadow priests have a significant advantage, since they can regenerate their mana much more quickly than their Discipline and Holy counterparts, allowing them to take the brunt of a few Mana Burns in order to remove the opposing healer from play. Disc and Holy priests should use the above techniques and stay as far away from Mana Burning Shadow priests as possible. Mana Burning is usually a relatively ineffective approach against Shadow priests.
Druids can shapeshift to prevent themselves from being Mana Burned. It is quite possible to pop into caster form for long enough to heal your target, and quickly shift back to Bear Form when you see the priest casting Mana Burn. This can be repeated indefinitely.
This ability was one of the demon hunter's abilities from Warcraft III. By channeling demonic energies into a bolt of negative energy the demon hunter is able to burn away an enemy's mana reserves.