The Grell is a beast found in the underdark. They typically stay together and although they can't see they have incredible senses of hearing and can track vibrations extremely well. They would be a good encounter for a party to explore more creative ways of escape aside from "I hit it with a hammer". The Grell may be a brain but they are not related to or used my mindflayers at all, of course you could always change that if you wished. This episode would be a good way to kick off an underdark campaign or give reason for your party to traverse the underdark for whatever reason. I left the actual item they're looking for and subsequent reward up to you. Thank you for listening and stay safe adventurers!!