Alpha Aethon

Innate Spellcasting. The Aethon has several spells granted by items it has devoured (See Magic Item Devourer), and can cast them without any components (Spell Save DC 17, +5 to hit with spell attacks):

At will: gate, plane shift,

1/Day: scorching ray

Magic Item Devourer. If the Aethon sees a magic item, it can devour it and gain its properties. Doing so takes 1 action for a common item, 1 minute for an uncommon item, 10 minutes for a rare item, 1 hour for a very rare or legendary item, or 1 day for an artifact. For example, if the Aethon finds a ring of three wishes, the Aethon can take 1 hour to devour it, and can then cast the wish spell three times. Each time the Aethon devours a legendary item or artifact, it grows one size and rerolls its hit dice, adding an additional die. This size goes up to huge, and for each time afterwards, the Aethon gains a hit die and improves one statistic by 2 or 2 statistics by 1, but must decrease its dexterity by 2. This can exceed 20, but not 30, and the Aethon's dexterity minimum is 3 (-4). This Aethon has eaten a headband of intellect, cubic gate, gauntlets of ogre power, iron flask, manual of bodily health, greatsword nine lives stealer, staff of striking, talisman of ultimate evil, tome of leadership and influence and a circlet of blasting. Upon its death, magic items it has devoured can be retrieved from its stomach by hacking through using an edged weapon. If an item has been swallowed within the year, it is fully intact, unless it is an artifact that must be destroyed in a specific way. It is otherwise dissolved. Weapons that have formed into tentacles, such as Nine Lives Stealer with this Aethon, can be retrieved by cutting off the tentacle they have formed within and bathing it in boiling water or lava for 1 hour. For the next five days, the weapon deals an extra 9 (2d8) fire damage.

Magic Resistance. The Aethon has advantage on saving throws to resist magical effects.

Magic Weapons. The Aethon's weapon attacks are magical.

Multiattack. The Aethon makes two attacks with Nine Lives Stealer or Staff of Striking, or makes one attack with each. It then makes a tentacle attack and a hook attack. It can make a beak attack in place of the tentacle and hook attacks. 

Tentacle. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage. The Aethon can also grapple (escape DC 19) the target. On the start of the target's turn while it is grappled, it takes 16 (2d8 + 7) bludgeoning damage. The Aethon has five tentacles that can grapple. When it grapples a creature with spell slots, the Aethon can choose to attach all tentacles to the spellcaster, curling itself into a ball around the creature. The escape DC increases to 24, the Aethon's armor class increases to 20 (natural armor), and attack rolls against the Aethon gain disadvantage. However, the Aethon can only take actions to release the spellcaster or move. When the spellcaster has been grappled for 1 minute, the creature must make a save with their spellcasting ability (DC 19). On a success, the spellcaster takes 35 (10d6) psychic damage and can escape from the grapple. On a failure, the creature looses all spell slots. These are transferred to the Aethon, who can now cast all spells known to the spellcaster, using a spell attack bonus of +14 and a spell save DC of 22. Charisma is its spellcasting ability.

Hook. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 20 (3d8 + 7) piercing damage plus 16 (3d10) poison damage.

Beak. Melee Weapon Attack. +14 to hit, reach 10 ft., one target. Hit: 40 (6d10 + 7) piercing damage.

Nine Lives Stealer (+3 Greatsword). Melee Weapon Attack. +14 to hit, reach 15 ft., one target. Hit: 21 (4d6 + 7) slashing damage. On a critical hit, the Aethon can expend one of nine charges to kill the target if they have less than 100 hit points.

Staff of Striking (+3 Quarterstaff). Melee Weapon Attack. +14 to hit, reach 15 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage. The Aethon can use up to 3 of its charges to deal an extra 3 (1d6) force damage on a hit.