Mephitis is the Champion of the Arachnid Spawn. She is about twice the size of a normal Giant Black Widow.
Mephitis is as most spiders quite a bit poisonous, her bite has a good chance of injecting a lethal poison, which will kill in a few seconds.
She will shoot a paralysing web at her attackers and then proceed to drag that web and whatever is stuck in it, back to where she is. Combine that with a huge melee damage and the lethal poison and it spells almost certain death.
Mephitis is one of the weaker champions when it comes to hit points, which means that she usually doesn't last long when there are a lot of players around.
When Mephitis is killed in Felucca, a Scroll of Power will be placed in the backpack of some of the players who damaged her. Additionally one of those who damaged her will also get a "Skull of Venom", which is one of the six skulls needed to summon the Harrower.