Grimnosh had golden green eyes and was eighty feet long from snout to tail. He could change color at will to blend in better with his surroundings.
Some time during or prior to 1364 DR, Grimnosh destroyed the village of Taskerleigh near Waterdeep. From Taskerleigh he retrieved the Morninglark harp, which he carried with him to his lair. Soon thereafter, he was approached by Iriador Wintermist ("Garnet"), who made a deal: if she could have the Morninglark, she would use it to issue a widely spread challenge for adventurers and riddlemasters to approach Grimnnosh with their best riddles. Grimnosh agreed and received a scroll from Garnet to give to anyone who succeeded at the challenge.
Soon, a group of mercenaries and adventurers led by Danilo Thann and Elaith Craulnober, accompanied by the riddlemaster Vartain, sought out Grimnosh in the High Forest. They failed to answer his riddle correctly, but nonetheless incapacitated him with magic and fled with the scroll. When Grimnosh finally recovered, he flew to Waterdeep to have his revenge on Garnet, plucking her from the crowd just as she was about to finalize a curse on the Harpers and the citizens of Waterdeep.