(Image courtesy of http://www.vanion.eu/news/3771/video-thron-des-donners-bosspreview-megaera/)

Megaera is the 5th encounter in the raid.

Megaera (according to WoW lore) is a corrupted Cloud serpent transformed into a hydra, hence the hydra style fight.

This fight requires

1. 2 Tanks
2. 3 Healers
3. 5 DPS

Overall Megaera has 315mil health (10-man), 789mil health (25-man). 

HOWEVER, you will never attack her body directly, you will have to "kill" her heads, which will in turn, deal damage to her body. 

Each head has ~45mil health (10-man), 112mil health (25-man) meaning you will have to "kill" a total of 7 heads to complete the encounter.

This is a 1-phase encounter of rotating heads.  There are 3 types of heads that you will encounter (4 in heroic). The are Flaming, Frozen, Venomous and Arcane (heroic only).  And obviously, they all come with their own unique set of abilties, both when they are AT the platform to be attacked, AND when they are in the concealing fog and cannot be hit.

The fight will start off with Frozen and Venomous heads up in front, and Flaming head in back casting Cinders on the raid.  Once you kill a head, it will retreat to the back, and the head in back will come to the front.  It is the raids job to rotate heads appropriately to split out the amount of abilities that the heads in front AND in back deal so dmg is balanced and doesnt kill the raid by being overwhelmed.  Note that for each head you kill there will be an additional head in the back of the same color dealing its extra ability.  So for example, the more red heads you kill, the more red heads that will be in the back casting cinders through the fight.

1. Flaming - When at the platform, it does a standard melee
   attack.  Its breath ability is called "Ignite Flesh".  This ability
   deals ~143-158k fire damage every second for 3 seconds.  It will place a
   debuff on affected players that deal additional 45k fire damage every
   second for 45 seconds.  This stacks, and will force tank swapping.
   • Cinders - used when in fog. Ignites a random plater,
     dealing 125k fire damage to them and an additional 55k every second for
     30 seconds, or until dispelled.  Players affected by Cinders can
     eliminate the "Icy Ground" effect from the Frozen heads (will discuss in
     next part).  When it is dispelled, it will drop a fire pool at the
     point of dispel.  This pool will inflict 125k fire damage to anyone in
     it every second they are in it.  These pools can be removed with one of
     the Frozen head abilities.
2. Frozen - Again, it has a standard melee attack.  Its breath ability is called "Artic Freeze".  It deals the same amount as the flame breath, but after receiving # stacks, the player will be frozen solid and stunned in place for 20 seconds.  (currently unknown as to how many stacks will cause the "frozen" effect)
   • Torrent of Ice -  used when in fog.  This is a beam ability (similar to Koralon) focuses a random player for 8 seconds and you have to kite it.  This beam will deal ~47-52k damage at the initial point of impact, and will also leave a fog called "Icy Ground" along where it is kited (this is what players affected by Cinders will want to run through to get rid of). The "Icy Ground" deals ~47-52k damage every 0.5 seconds to any person is in it (unless affected by cinders), and reduces movement speed by 5% per stack.  At 10 stacks the player becomes a frozen in place.  When a player is focused by this ability, they will want to kite it over any Fire pools that are up as they will cause the pool to despawn once the ice beam hits it.
3. Venomous - Again, standard melee attack.  Its breath is called Rot Armor.  This deals the same damage as the other breaths and causes the affected players to take an additional 10% damage from all sources per stack for 45 seconds. Again, forcing a tank swap.
   • Acid Rain - Used when in fog.  This ability will spit a globule of acid at a random player, dealing 350k nature damage to ALL players upon impact.  The damage dealt by this decreases the further you are away from the point of impact.
4. Arcane (Heroic) - See heroic guide

Rampage - Megaera uses this any time a head is killed.  Its just a massive AoE shitstorm on the raid.  After a head is killed, have the raid stack in one location and use Def CDs (more important later in the fight) and heal through it.

Hydra Frenzy - Whatever head is NOT killed once they begin rotating will reappear with this buff.  This buffs brings its health back to full when it reappears and gives it a 20% haste increase.  Causing all heads in melee to melee faster, and all heads of that color in the back to use their abilities faster.

Elemental Blood of Megaera - For each head that is killed, additional heads of that same color will receive a 15% increase the damage they deal.

Megaera's Rage (raid wipe mechanic) - Whenever there is nobody in melee to attack the heads, the heads will use "Megaera's Rage", and the heads will cast elemental bolts.  These will deal ~240-255k elemental damage to all players within 5 yards of the impact locations.

This is going to be one of those fights with a lot of stuff going on and will take getting used to.  Just have to keep rotating head kills so they dont get high stacks of the "Frenzy", and use the proper abilities to negate the Icy Ground and Cinder Pools.