Lethon was one of the Emerald Dragons. The other Emerald Dragons were Emeriss, Ysondre, and Taerar. These dragons were world spawns located in Duskwood (Twilight Grove), the Hinterlands (Seradane), Feralas (Dream Bough), and Ashenvale (Bough Shadow). They were raid-level difficulty. However, some dragons were more challenging than others. Lethon was considered to be the hardest, although this was largely a popular myth. The Shadow Bolt Twirl was deadly if not handled well but if the raid had a capable Main Tank who knows what to do, Lethon became one of the easier outdoor bosses.
All four dragons shared the Sleep (summoning "Dream Fog"), Noxious Breath, Tail Sweep, Mark of Nature and Aura of Nature abilities. Each dragon also had a few unique abilities. One of these were considered the dragon's ace move, and were performed when the dragon lost 25 ± 5% of its max life. In other words, the ace move was only performed three times; when the dragon was at 25%, 50%, and 75% life. When it is performed, the dragon also yelled something.
Used by all the Dragons of Nightmare
Seeping Fog - Summons two clouds of Dream Fog that will chase random players around the dragon and put them to sleep for 4 seconds.
Noxious Breath - Deals 3000 Nature damage and applies a 30 second DoT that increases cooldowns on all abilities by 10 seconds and inflicting 350 to 450 Nature damage every 3 seconds. The DoT can stack up to 6 times. The main tank should be the only one affected by this.
Tail Sweep - Inflicts 925 to 1075 damage on enemies in a 30 yards cone behind the dragon, knocking them back.
Summon Player - Teleports the player who has the most aggro in front of the dragon if the player tries to run out of combat. This ability has first been noted after patch 2.0.1.
[Mark of Nature] - If players are killed by a nightmare dragon, they will be afflicted by a 15 minute debuff called Mark of Nature. If engaging the dragon during this time, they will be slept for 2 minutes rather than 4 seconds as with the Dream Fog.
[Aura of Nature] - A invisible pulsing aura that interrupts actions like eating, drinking and bandaging when close to the dragon. Will trigger the 2 minute sleep when combined with Mark of Nature.
All the Dragons of Nightmare are also immune to nature damage.
Abilities exclusive to Lethon
Shadow Bolt Whirl - Fires four sets of shadow bolts at everyone on one side of him, then alternates to the other side, dealing 800-1200 damage. On each volley his feet will glow.
25% ability: Draw Spirit - Everyone within 100 yards of Lethon will be stunned for 5 seconds and a Spirit Shade will walk out of their body. While stunned, each player will take 657 to 843 shadow damage every 2 seconds. The shades will walk toward Lethon and will heal him if they reach him. Each shade will heal Lethon for 15000 HP. The shades have 500 health each and are immune to AoE spells.
Clump the raid on one side. Pull Lethon. The Tank should be tanking Lethon next to one of the two pillars of the temple. Lethon only shoots shadow bolts on one side of him, so it is important that the tank recognizes the side that Lethon is shooting the bolts in and moves Lethon's head to the correct pillar. Running through him to the next pillar is the most efficient way of turning Lethon. A good way to know when the MT has to turn Lethon over is to have one raid member count the number of times his claws glow. After four glows, the Shadow Bolt Volleys change the side they shoot to.
At +/- 10% of 75%, the raid healers should be ready to top the main tank off. Overhealing the tank, although bad on the healing meters, can make the difference between a tank surviving through the banish, or not surviving through it. Keep 3 heal over times on the main tank at all times at the 85%, 60%, and 35% marks. Make sure DPS kills as many shades as possible to prevent Lethon from getting healed. However, killing the shades is not that necessary, as healers can run out and drink if needed, and the only real challenging parts to this fight is when everyone is banished.
Not a vital note for the fight, but the Noxious Breath DoT persists on the MT after Lethon dies, it's rather funny when a guild begins to celebrate and 5 seconds later the MT falls over dead; just remember to heal him after Lethon dies