Bob's original stat block, verbatim. A ravenous, gluttonous undead spirit, currently locked away alongside the Ice Lich.
Any living creature within a 60-ft radius must make a DC 28 Fortitude save at the start of their turn or take 2d6 points of Constitution drain and become Exhausted from sudden extreme famine.
When the Hollow Maw hits with its bite, the target must succeed on a DC 28 Fortitude save or suffer 2d4 permanent Strength and Constitution drain. The Hollow Maw heals 10 hit points for each point drained.
Can swallow a Medium or smaller opponent it has grappled. The inside of its gullet is a planar void: targets take 4d8 crushing damage plus 4d8 negative energy damage per round. Escaping requires dealing 40 points of damage to the gullet (AC 22) with a light slashing weapon.
Claws drain all fluid and moisture from the target, dealing 2d6 points of Desiccation damage that bypasses standard resistances.
As an immediate action, can swallow a targeted spell or magical ray aimed at it, negating the effect and gaining temporary hit points equal to 5x the spell level.
Fast Healing 15, Damage Reduction 15/Epic and Good, Spell Resistance 35.
Cold, Acid, Poison, Paralysis, Stunning, and Polymorph.
Immune to any effect that restricts movement or banishes entities back to native planes.
A from-scratch monster conversion, built to 5e's own math for a solo, CR 21 threat rather than forcing 3.5e's epic-grappler numbers into a 5e frame.
Intimidation, Perception, and Stealth use Expertise (double proficiency), the closest 5e equivalent to 3.5e's towering skill bonuses. Tremorsense covers its Burrow speed; the polymorph immunity from the original block is folded into the Unchanging Hunger trait below.
At the start of each of its turns, the Hollow Maw deals 10 (3d6) necrotic damage to one creature of its choice within 60 feet of it. That creature also gains one level of exhaustion unless it succeeds on a DC 22 Constitution saving throw.
The Hollow Maw regains 15 hit points at the start of its turn if it has at least 1 hit point (reskin of Fast Healing 15).
The Hollow Maw has advantage on saving throws against spells and other magical effects (reskin of Spell Resistance 35).
Resistant to bludgeoning, piercing, and slashing damage from nonmagical attacks (reskin of Damage Reduction 15/Epic and Good).
The Hollow Maw can't be polymorphed, banished, or have its speed reduced by any effect (reskin of Inescapable Hunger).
If the Hollow Maw fails a saving throw, it can choose to succeed instead.
The Hollow Maw makes one Bite attack and two Claw attacks.
Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 21 (3d8+8) piercing damage. If the target is a creature, it must succeed on a DC 22 Constitution saving throw or its Strength and Constitution scores each go down by 1d4. The Hollow Maw regains hit points equal to the total drained. The target dies if this reduces either score to 0.
Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 13 (2d6+8) slashing damage plus 7 (2d6) necrotic damage as it desiccates the wound. This necrotic damage can't be healed by any means until the target finishes a long rest.
The Hollow Maw makes one Bite attack against a Large or smaller creature it is grappling. If the attack hits, that creature is swallowed instead of taking damage, and the grapple ends. A swallowed creature is blinded and restrained, has total cover against attacks from outside the Hollow Maw, and takes 21 (6d6) necrotic damage at the start of each of the Hollow Maw's turns. The Hollow Maw can have only one creature swallowed at a time. If it takes 30 damage or more on a single turn from a creature inside it, it must succeed on a DC 20 Constitution saving throw or regurgitate the creature, which falls prone in a space within 10 feet. If the Hollow Maw dies, a swallowed creature is no longer restrained and can escape by using 15 feet of movement to exit.
When targeted by a spell or a magical ranged attack, the Hollow Maw can use its reaction to negate that spell or attack. It gains temporary hit points equal to five times the level of the negated spell (or 5 temporary hit points for a magical ranged attack that isn't a spell).
The Hollow Maw can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. The Hollow Maw regains spent legendary actions at the start of its turn.
The Hollow Maw makes one Claw attack.
The Hollow Maw burrows up to half its burrow speed without provoking opportunity attacks.
The Hollow Maw makes one Bite attack.