Ancient Amoeba

Transparent. Even when the amoeba is in plain sight, it takes a successful DC 15 Wisdom (Perception) check to spot it if it has neither moved nor attacked. If it is underwater, this check is made at disadvantage. A creature that tries to enter the amoeba’s space while unaware of the amoeba is surprised by it.

Amphibious. The amoeba can breathe both air and water.

Pseudopod. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 16 (3d8 + 3) bludgeoning damage and 21 (6d6) acid damage and make a DC 18 Dexterity saving throw or be grappled. Until the grapple ends, the target is restrained.

Engulf. A creature that the amoeba has grappled must make a DC 18 Strength saving throw. If the target fails, it takes 21 (6d6) acid damage and is engulfed. An engulfed creature can’t breathe, is restrained, and takes 34 (8d6) acid damage at the start of each of the amoeba turns. An engulfed creature can try to escape by taking an action to make a DC 18 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the amoeba.