Xaril the Poisoned Mind was one of the Klaxxiparagons. He was an expert alchemist who specialized in using poisons and explosive compounds. He was awakened by the Wakener to combat the sha threat. Along with the rest of the Klaxxi, he later sided with Garrosh Hellscream, who had obtained the Heart of Y'Shaarj, against the Alliance. He fought in the Underhold during the Siege of Orgrimmar, where his brethren and him were eventually killed.
There are many wonderful poisons, Wakener. Some can boil you alive. Some will eat your flesh. Some burst you open from the inside.
So many wonderful varieties.
Have you ever tried drinking poison, Wakener? You must start with something very mild, only a few drops at first.
You will likely become violently ill. Perhaps you will even lose an antennae or two.
But do it long enough, and you will become quite used to it.
I was once the Empress Shek'zeer's most prized royal alchemist, peerless, and above all others.
Pah! I remember when Un'sok was still a drone, afraid to mix his first toxin. I dare him to face me alone. Then we shall see whose alchemy is supreme.
Gossip after released
I am made anew!
Before I was cast in amber, I made a great gamble.
I realized the amber could perform a miracle that I would never achieve, not even with a thousand years of careful experimentation.
Before I became a paragon, I injected myself with a dozen lethal poisons. The life-giving properties of amber protected me as I guessed, and time did the rest.
The blood in my veins is now thick with toxins. I shall fear no poison ever again. Truly, I am my own masterpiece!
Xaril the Poisoned Mind says: How does it feel to go rogue a second time, swarmkeeper? Your empress was also gripped by paranoia, was she not?
Hisek the Swarmkeeper says: Grand Empress Zek'hara masked her true intentions behind the guise of matronly worry. She claimed her overbearing ways were necessary to protect us.
Hisek the Swarmkeeper says: When she tried to forbid the swarm, it was to prevent, as she put it, "a senseless loss of life."
Hisek the Swarmkeeper says: In reality, she was obsessed with control. She aspired to stay in power long past her time, and was willing to sacrifice our eternal cycle to achieve that. We had no choice but to end her.
Hisek the Swarmkeeper says: How does it feel? I have no regrets, if that is what you are asking. I am honored to serve the will of the Klaxxi.
Hisek the Swarmkeeper says: And having seen what Grand Empress Shek'zeer is doing to our land... I need no convincing.
Xaril the Poisoned Mind says: Of course.
Xaril the Poisoned Mind says: I served Shek'zeer for a time, you know. Before I was preserved. Her madness is nothing so rational as ambition.
Xaril the Poisoned Mind says: She trusts no one. Paranoia gnaws at her mind. She is driven by fear unrelenting.
Klaxxi enhancement
I have coated your weapons with a special serum. Use it to inject your enemies. Too many injections, and their systems will overload.
This section contains information exclusive to Hearthstone and is considered non-canon.
Xaril appears as a legendary card for the Rogue class in the Whispers of the Old Gods expansion for Hearthstone. His flavor text reads: "It's basically your own fault if you go around drinking weird green potions handed out by creepy mantid dudes."
Xaril the Poisoned Mind
[30-35] Beneath the Heart of Fear 
[Blade of the Poisoned Mind], a lost treasure found in the Dread Wastes, belonged to him.
Patch 5.0.4 (2012-08-28): Added.