Beast II, Tiamat

Legendary Resistance (3/day): If Tiamat fails a saving throw, it can choose to succeed instead.

Magic Resistance: Tiamat has advantage on saving throws against spells and other magical effects.

Siege Monster: Tiamat deals double damage to objects and structures.

Primordial Sea: A sea of primordial mud surrounds Tiamat. For each minute she spends on the primordial plane, the sea’s radius grows by 5 ft. Aberrations in contact with the mud heal 8d6 HP at the start of their turn, while all other creatures take the same amount of Necrotic damage. Tiamat has tremorsense in her Primordial Sea. The sea remains if Tiamat is removed, shrinking at the same rate it grew.

Mother to All: If a humanoid’s corpse enters the radius of Tiamat’s Primordial Sea, it rises again as an aberration in service to Tiamat at the start of her turn. If the creature it used to be had any class levels it uses the original’s stat block, but is an aberration instead of any other creature type. Otherwise, it turns into a Lahmu.

Self-Manifestation: As long as Tiamat exists on the Material Plane, any effect that would kill her or reduce her to 0 HP instead reduces her to half her total health. If her physical form would die and it is not on the Astral Plane, her spirit returns to the Astral Plane to reconstitute herself over time.

Nega-Genesis: Whenever Tiamat would take damage from a spell or magical effect, roll a d20. On a 6 or higher, halve the damage.

Multiattack: Tiamat can use its Frightful Presence, then it makes four attacks: 1 bite attack, 2 claw attacks, and one horn attack.

Bite: Melee Weapon Attack: +14 to hit, 10’ reach, 1 target. Deals 4d12+6 (32) Piercing damage and grapples the creature (DC 20 escape) until the grapple ends, the target is restrained and Tiamat cannot bite another creature.

Claw: Melee weapon attack: +14 to hit, 15’ reach, 1 target. deals 4d8+6 (24) slashing damage.

Horns: Melee weapon attack: +14 to hit, 10’ reach, 1 target. deals 4d10+6 (28) piercing damage

Frightful Presence: Each creature of Tiamat’s Choice within 120’ of it and aware of it must succeed on a DC 23 Wisdom save or become frightened for one minute. A creature can repeat the save at the end of each of its turns, with disadvantage if Tiamat is within line of sight, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to Tiamat’s Frightful Presence for the next 24 hours.

Tiamat can take three legendary actions, choosing from the options below. Only one can be used at a time and only at the end of another creature’s turn. Tiamat regains spent legendary actions at the start of its turn.

Attack: Tiamat makes one claw attack or horn attack.

Move: Tiamat moves up to half its speed.

Chomp (2 actions): Tiamat makes one bite attack.

Sonic Scream (Costs 3 actions): Each creature of Tiamat’s choosing that can hear Tiamat must make a DC 18 Constitution save, taking 5d6+6 Thunder damage on a failed save or half as much on a success. Any concentration saves caused by this damage are done at a disadvantage.