"The blood shall be a sign for you on the houses where you live; and when I see the blood I will pass over you, and no plague will befall you to destroy you when I strike the land..."
The lair can be found in the far south of Saphery on the Order side. The lair can be found around 48000,61000. To enter, you must first find a level 40 Champion Sprite named Arixxis the Scout and kill him. Someone in the killing party will get Grimshimmer's Ley stone that can be right-clicked on to open a Grimshimmer Portal. Arixxis the Scout is easy to spot because it keeps exploding and vanishing as it flies through the air to the north of the lair. After entering the portal, you will be on a ledge with a tower full of Grimgyre Harpies and Grimgyre Sprites which are all level 40 Champions. You must fight your way through these to reach the Hero of the Lair.
Lady Grimgyre herself starts appearing to be a High Elf, but at around 20-30% health she transforms into a Sorceress.