Snake Queen's Lair

From publisher blurb: Battle a marilith alongside her demonic and snake minions in this boss battle! Lair of the Snake Queen is a Fifth Edition encounter for four characters of 4th, 10th, or 16th level. This encounter is setting and adventure independent. It can easily be inserted into any story that you like, or even run as a one-shot. The add-on includes rules for running the featured marilith independently or as part of a multi-wave battle.