Ooze Cube

The creature takes up its entire space. Other targets can enter the space, but a target that does so is subjected to the creature's Engulf action and has disadvantage on the saving throw. Targets inside the creature can be seen but have total cover. A target within 5 feet of the creature can take an action to pull a target or object out of the creature. Doing so requires a successful Strength check (DC = 8 + proficiency bonus + Strength modifier), and the target making the attempt takes X acid damage. The creature can hold only one Large creature or up to four Medium or smaller creatures inside it at a time. (Gelatinous Cube, pp. 242, MM)