Blood Blob

Amorphous. The blood blob can move through a space as narrow as 1 inch wide without squeezing.

Berserk. When the blood blob starts its turn with 40 hit points or fewer, roll a d6. On a 6, the blob goes berserk. On each of its turns while berserk, the blob attacks the nearest creature it can see. If no creature is near enough to move to and attack, the blob attacks an object, with preference for an object smaller than itself. Once the blood blob goes berserk, it continues to do so until it is destroyed or regains all its hit points.

Space Hog. The blood blob takes up its entire space. Other creatures can enter the space, but a creature that does so is subjected to the blood blob’s Engulf and has disadvantage on the saving throw. Creatures inside the blood blob can’t be seen and have total cover. A creature within 5 feet of the blood blob can take an action to pull a creature or object out of the blob. Doing so requires a successful DC 12 Strength check, and the creature making the attempt takes 10 (3d6) necrotic damage. The blood blob can hold only one Medium or smaller creature inside it at a time.

Blood Splatter. Any time the blob is hit in combat, a gout of blood erupts from its body. All creatures within 10 feet of the blob must succeed on a DC 15 Dexterity saving throw or take 3 (1d6) necrotic damage be blinded until the end of the creature’s next turn. On a successful save, a creature takes only half damage and is not blinded. 

Immutable Form. The blob is immune to any spell or effect that would alter its form.

Multiattack. The blob makes two bloody tendril attacks.

Bloody Tendril. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage, and the target must make a DC 15 Constitution saving throw. On a failed save, the target takes 22 (4d8 + 4) bludgeoning damage plus 14 (3d6 + 4) necrotic damage; its hit point maximum is reduced by the amount equal to the necrotic damage, and the blood blob regains hit points equal to that amount. The reduction in the target’s hit point maximum lasts until the target finishes a long rest. The blob's target dies if this reduces its hit point maximum to 0.

Engulf. The blood blob moves up to its speed. While doing so, it can enter Large or smaller creatures’ spaces. Whenever the blood blob enters a creature’s space, the creature must make a DC 12 Dexterity saving throw. On a successful save, the creature can choose to be pushed 5 feet back or to the side of the blood blob. A creature that chooses not to be pushed suffers the consequences of a failed saving throw. On a failed save, the blood blob enters the creature’s space, and the creature takes 10 (3d6) necrotic damage and is engulfed. The engulfed creature can’t breathe, is restrained, and takes 21 (6d6) necrotic damage at the start of each of the blood blob’s turns. When the blood blob moves, the engulfed creature moves with it. An engulfed creature can try to escape by taking an action to make a DC 12 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the blood blob.