A large level 43 spider. Found near one of the 3 underground entrances near the coords listed, in Mordant Knolls in Droughtlands. It's accompanied by several other lesser spiders, and spawns adds during the encounter.
NOTE: This is the 'Mordant Queen' rare from the achievement lists. 'Mordant Widow' is the actual name of the rare as it spawns. If you're using a targeting macro, make note of this as well...being that every spider is 'Mordant something-or-other', you'll want to add WIDOW, not QUEEN to the name.