Belara'grast

Amphibious: Balara'grast can breathe air and water

Legendary resistance (3/day): If the dragon fails a saving throw, it can instead choose to succeed

Legendary accuracy (3/day): If the dragon misses with an attack, it can instead choose to hit

Necrotic Absorption: Magic which would restore hitpoints to a creature, for example, healing word, instead damage Belara'grast; conversely, when the dragon would suffer necrotic damage, he instead restores hitpoints for the same amount. 

Detection Protection: Belara'grast is immune to being detected with divination magic, and attempts to identify or detect undead do not identify him. 

Spellcasting: the dragon can cast the following spells without requiring material components: 

• Fog Cloud, Inflict Wounds, Hold Person, Counterspell, Major Image (2/day, each)
• Cloudkill, Circle of Death (1/day, each)

The dragon may multiattack, cast a spell, or take the dash or dodge actions using its action. 

Multiattack: Belara'grast makes two tentacle attacks, and one bite attack

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target.
Hit: 22 (3d10 + 7) piercing damage, plus 13 (3d8) necrotic damage. The target’s hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampiric dragon regains hit points equal to that amount. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Tentacle. Melee Weapon Attack: +14 to hit, reach 20 ft., one target.
Hit: 19 (3d8 + 7) bludgeoning damage, and the target is grappled (escape DC 20). As part of this attack, Belara'grast can pull the target 10 feet towards itself. Until this grapple ends, the target is restrained, and the vampiric dragon can’t use that tentacle on another target. The vampiric dragon has eight tentacles.

Dark Ink Cloud (Recharge 5–6). While underwater, the dragon expels a 60-foot-radius cloud of black ink. The area is heavily obscured for 1 minute, except if the water is highly turbulent in which case strong currents might disperse the ink sooner than this. The dragon can see through the cloud, and creatures within it have disadvantage on attack rolls. At the start of each of the dragons turn, all creatures within the cloud, including Belara'grast, suffer 2d12 necrotic damage.

Vampiric Resilience: If Belara'grast is reduced to 0 hit points, it can use its reaction to deal 3d12 necrotic damage to all creatures within 15 feet of it. It is not rendered unconscious and instead remains on a number of hit points equal to the necrotic damage dealt. 

While underwater, Belara'grast 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. The dragon regains spent legendary actions at the start of its turn.

• Tail Slap. The dragon makes one swiping attack with its tail (Melee Weapon Attack: +14 to hit, reach 30 ft., one target. Hit: 19 (3d8 + 7) bludgeoning damage, the target must succeed on a DC18 dexterity saving throw or be knocked back 20 feet. 
• Dark Mist Step. The vampiric dragon teleports up to 60 feet to an unoccupied space it can see.
• Life Drain (Costs 2 Actions). The vampiric dragon forces one creature it has grappled to make a DC 21 Constitution saving throw. On a failed save, the target takes 28 (8d6) necrotic damage, and the vampiric dragon regains hit points equal to the damage dealt. if this attack reduces a creature to zero hit points, it is swallowed.