Aspect of Night. The Nightlords have a variety of varied abilities, many of which cause saving throws. For simplicity, all saving throws mentioned that are caused by Adel have a DC of 23. Additionally, its attacks count as magical.

Sickness Prone (3/Day). Adel has disadvantage on saving throws to resist being poisoned. Additionally, Adel cannot use its legendary resistance to resist being poisoned. When Adel becomes poisoned, it must spend its entire turn to vomit it out. This poisoned vomit spreads outwards from Adel, 30 ft. in all directions. Creatures ending their turn in it take 20 (8d4) poison damage and must succeed a Constitution saving throw or become poisoned. After Adel vomits, it loses the poisoned condition.

Hidden Lightning.  When Adel reaches half hit points it stays in place for a turn, stomping around in frustration. It must spend its entire turn to use the Bring Forth Lightning action, unless it has been poisoned.

Night Aspect: Devour. When Adel reduces a creature to zero hit points, it regains 15 hit points. 

Legendary Resistance (3/Day). If Adel fails a saving throw, it can choose to succeed instead.

Multiattack. Adel makes 2 attacks, one with its slam, and one with its bite.

Slam. Melee Attack Roll: +10 to hit, reach 10 ft., 1 target. Hit: 27 (6d6 + 6) bludgeoning damage. Hit targets must succeed a Dexterity saving throw or fall prone.

Bite. Melee Attack Roll: +12 to hit, reach 10 ft., 1 target. Hit: 30 (8d6 + 2) bludgeoning damage.

Night Aspect: Divest Essence. After Adel uses its Night Aspect: Devour trait, it can choose to use this action to gain even more hit points. It targets the creature reduced to zero hit points, and completely devours it, gaining 30 (8d6 + 2) hit points. The absorbed creature can no longer be brought back to life, excluding through use of the wish spell.

Bring Forth Lightning (Via Hidden Lightning). Adel stomps around, growing into a fit of rage. Adel's skin bursts with crackling purple lightning, as the surroundings come to grow a similar hue. All creatures within 90 ft. are forced to make a Dexterity saving throw, taking 35 (10d6) lightning damage on a failed save, or half as much on a success. Then, Adel uses its rotting limbs and wings to leap into the air, just to smash back down to earth. Any creatures touching the ground within 120 ft. must make a Dexterity saving throw. On a fail, they take 70 (20d6) lightning + 17 (5d6) bludgeoning damage. Until Adel chooses to end its fit of rage, all attacks made after the use of this action will deal an additional 2d6 lightning damage.

Minor Fissure (Bring Forth Lightning required). Adel ruptures the ground around it, forcing all creatures touching the ground within 30 ft. to make a Dexterity saving throw. On a fail, they take 26 (8d6) lightning + 7 (2d6) bludgeoning damage, or half as much on a success.

Shocking Multiattack (Bring Forth Lightning required). Adel makes three attacks, one with its bite, one with its slam, and one minor fissure.

Channel the Night. (2/Day). Adel channels the power of the Nightlord. For 2 turns, it is immune to lightning and acid damage, and it no longer has disadvantage on resisting poison.

Stress Wings. Upon being attacked in melee, Adel can use this reaction to add 30 ft. of flying speed to its movement until the end of its next turn.

Adel 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. Adel regains spent legendary actions at the start of its turn. 

Shift. Adel moves 60 ft. in a straight line.

Attack. Adel uses either it bite, slam, or minor fissure if available.

Night Aspect: Recharge (Costs 3 Actions). Adel over exerts itself, taking the Bring Forth Lightning action again as long as it already has within the day. After this legendary action is used, Adel cant use legendary actions again for 7 days.