The portal is inside the drop of Mhoire Castle Northeast Tower. You must have completed the three trials and spoken to Shade of Antemy to gain access. This is one of the few dungeons with creatures that drop Tier 7 Loot. The creatures also drop Pristine Mana Shards, Jester's Tokens, Pyre Shrouds, Mhoire Castle Treasury Keys, Ornate Bone Keys and Pyre Skeleton Jaws. The Wight Blade Sorcerers count toward your faction's Wight Blade Sorcerer Kill Task. The respawn timer is long.