Ysondre is one of the Emerald Dragons. The other Emerald Dragons are Lethon, Emeriss, and Taerar. These dragons are world spawns located in Duskwood (Twilight Grove), the Hinterlands (Seradane), Feralas (Dream Bough), and Ashenvale (Bough Shadow). They are raid level difficulty. However, some dragons are more challenging than others. Ysondre is considered to be the easiest.
All four dragons share the Sleep (summoning "Dream Fog"), Noxious Breath, Tail Sweep, Mark of Nature and Aura of Nature abilities. Each dragon also has a few unique abilities. One of these is considered the dragon's ace move, and will be performed when the dragon loses 25 ± 5% of its max life. In other words, the ace move will only be performed three times; when the dragon is at 25%, 50%, and 75% life. When it is performed, the dragon will also yell 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 Ysondre
Lightning Wave - Lightning bolt that strikes a random target for 463 to 537 Nature damage, then arcs to other nearby players. The spell affects up to 10 targets, inflicting greater Nature damage to each successive target.
25% ability: Summon Demented Druid Spirits - Summons several druids with low health (2,442 HP) that attacks the raid. While easily killed, a spirit will after 10 minutes die by itself. The spirits look like translucent green-shining night elves and tauren clad in druid-like armor. Aside from melee, each spirit has three abilities:
Curse of Thorns - Gives the target a 50% chance to take 38 to 82 damage on each attack when attacking something. Lasts for 3 minutes.
[Moonfire] - Burns the target for 219 to 281 arcane damage and then applies a DoT which deals 88 to 112 arcane damage every 3 seconds for 12 seconds.
Silence - Prevents the target from casting spells for 5 seconds.
The strands of LIFE have been severed! The Dreamers must be avenged!
When summoning Druids
Come forth, you Dreamers - and claim your vengeance!
It is interesting to note that although the other dragon's quotes have words with negative effects ("DISEASE", "NIGHTMARE" and "SHADOW") in capitals, Ysondre's word "LIFE" bears a strong meaning to druids and dragons alike.
While the eyes of Lethon, Emeriss and Taerar are wide open, Ysondre's eyes are closed. However with the anniversary encounter, her eyes are open.