Bogrodar

Necrotic Absorption. Whenever the shambling mound is subjected to necrotic damage, it takes no damage and instead regains a number of hit points equal to the necrotic damage dealt.

Siege Monster. Bogrodar deals double damage to objects and structures.

Heart of Darkness. When Bogrodar's Engulf feature deals necrotic damage, half the total necrotic damage dealt is also taken by every other creature within 60 feet of the mound. Creatures cannot take necrotic damage from Bogrodar's engulf more than once per turn, and Bogrodar does also take the damage. 

Blackened Furnace. If Bogrodar has at least one creature engulfed its speed increases by 10 feet, and if it has two creatures engulfed it can use its Rage of the Starved action without restriction.

Thick Skinned. Whenever Bogrodar fails a saving throw, it can choose to take 40 damage to succeed on the saving throw instead.

Multiattack. The shambling mound makes two slam attacks. If both attacks hit a Medium or smaller target, the target is Grappled (escape DC 18), and the undead shambling mound uses its Engulf on it.

Slam. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) bludgeoning damage.

Engulf. The shambling mound engulfs a Medium or smaller creature Grappled by it. The engulfed target is Blinded, Restrained, and unable to breathe, and it must succeed on a DC 17 Constitution saving throw at the start of each of the mound's turns or take 11 (1d10 + 6) bludgeoning damage plus 13 (3d8) necrotic damage. If the mound moves, any engulfed targets move with it. The mound can have up to two creatures engulfed at a time. 

If the mound takes 20 damage or more on a single turn from a creature inside it, the mound must succeed on a DC 19 Constitution saving throw at the end of that turn or release all engulfed creatures, each of which falls Prone in a space within 10 feet of the mound. If the mound dies, any engulfed creature is no longer Restrained by it and can escape from the corpse using 10 feet of movement, exiting Prone.

Rage of the Starved. If Bogrodar has taken its action but hasn't engulfed a creature this turn, it may spend a bonus action to move up to its speed and make a Slam attack. This movement does not draw opportunity attacks.

Abandon the Body. As a reaction to being reduced to less than 20 hit points, the undead root-stem that is Bogrodar's true form can attempt to flee. It is large size, has 20 hit points, a walking and burrowing speed of 15 feet, an AC of 10, and immunity to necrotic and poison damage. The stem leaves a small size tunnel where it burrows, and can fit comfortably into a small or larger space. The stem's retreat is obvious and not hidden, and at the start of Bogrodar's next turn its mound body detonates. Any creatures within 30 feet of the body must make a DC 17 Constitution saving throw, taking 33 (10d6) necrotic damage on a failure, or half as much on a success.

Bogrodar can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Bogrodar regains spent legendary actions at the start of its turn.

Strike. Bogrodar uses its Slam attack.

Greed (Costs 2 Actions). Bogrodar suppresses the effects of a magical item within 90 feet of it. This suppression lasts until Bogrodar dies, releases it, or suppresses another item.

Desiccating Blast (Costs 2 Actions). Bogrodar releases a blast of necrotic energy in a 10 foot wide, 60 foot long line. All creatures with that area must make a DC 17 Constitution saving throw or take 44 (8d10) necrotic damage, or half as much damage on a success.