Grenwog was the easter challenge monster, He could be fought by typing "/join grenwog" It is inside the mountains, between the Ruins and the Citadel. Grenwog isn't going to be the only mystery that is inside the mountains.
Location: Grenwog
Level: 15
HP: 30000
MP: 30
Gold:
Drops:Grenwog Tooth,Mystery Egg,Mini Grenwog