Armored Horror

Acidic Innards. Any creature that hits the horror with a melee attack takes 9 (2d8) acid damage.

Reformation. When the horror is reduced to 0 hit points, its body melts into a gelatinous ooze that begins to move together at a speed of 5 feet per round. After 1d10 rounds, the ooze reforms, turning into an adult oblex.

Siege Monster. The horror deals double damage to objects and structures.

Spider Climb. The horror can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Trampling Stride. The first time the horror enters another creature’s space on its turn, that creature must succeed on a DC 15 Strength saving throw or be knocked prone.

Multiattack. The horror uses its Maddening Presence and makes two attacks: one with its Stab and one with its Tendrils.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 20 (4d6 + 6) piercing damage plus 7 (2d6) acid damage.

Maddening Presence. The horror targets one creature it can see within 30 feet of it. If the target can see or hear the horror, the target must make a DC 15 Wisdom saving throw. On a failed saving throw, the target gains a Short Term Madness. If the saving throw fails by 5 or more, the target gains a Long Term Madness. If a creature's saving throw is successful, the creature is immune to the horror's Maddening Presence for the next 24 hours.

Stab. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 6) piercing damage.

Swallow. The horror makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the horror, and it takes 21 (6d6) acid damage at the start of each of the horror's turns. A horror can have only one creature swallowed at a time.

If the horror takes 20 damage or more on a single turn from the swallowed creature, the horror must succeed on a DC 17 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the horror. If the horror dies, a swallowed creature lands prone and in the ooze, taking 14 (4d6) acid damage, and is no longer restrained.

Tendrils. Melee Weapon Attack: +10 to hit, reach 20 ft., one creature. Hit: 7 (2d6) acid damage and the target is grappled (escape DC 15). Until the grapple ends, the target is restrained and has disadvantage on Strength checks and Strength saving throws. The horror can have only one creature grappled at a time. 

Reel. The horror pulls a creature grappled by it up to 20 feet straight toward its mouth.

Skittering Retreat. In response to taking damage, the horror skitters 15 feet in any direction. When it does this, opportunity attacks against the horror have disadvantage.