Colossal Owl

Flyby. The owl doesn't provoke opportunity attacks when it flies out of an enemy's reach.

Innate Spellcasting. The owl’s innate spellcasting ability is Wisdom (spell save DC 27, +19 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: alarm, animal friendship (as a 3rd-level spell), animal messenger (as a 9th-level spell), beast sense, catnap (as a 5th-level spell), charm person (as a 5th-level spell), clairvoyance, comprehend languages, dancing lights, darkvision, detect evil and good (no concentration), detect poison and disease (no concentration), dimension door, druidcraft, enhance ability (owl’s wisdom, no concentration), faerie fire, feather fall, fog cloud (as a 3rd-level spell, no concentration), glyph of warding, guidance, healing spirit (as a 3rd-level spell), hold monster, hold person (as a 6th-level spell), intellect fortress (no concentration), lesser restoration, locate animals or plants, major image (no concentration), mind sliver, mind spike (no concentration), minor illusion, nondetection, Raulothim's psychic lance, see invisibility, sending, silent image (no concentration), sleep, Tasha's mind whip, tongues, true seeing

3/day each: charm monster (as a 5th-level spell), compulsion, confusion, detect thoughts (no concentration), greater invisibility (no concentration), hold monster (no concentration), hypnotic pattern, locate creature, moonbeam (as a 5th-level spell), scrying, summon beast (as a 5th-level spell), wrath of nature (no concentration)

1/day each: dominate person (as an 8th-level spell), dream, druid grove, geas, greater restoration, hallucinatory terrain, major image (as a 6th-level spell), mass suggestion (as a 7th-level spell), programmed illusion, sleep (as a 9th-level spell), symbol (fear, sleep, or stunning only)

Keen Hearing and Sight. The owl has advantage on Wisdom (Perception) checks that rely on hearing or sight.

Legendary Resistance (4/Day). If the owl fails a saving throw, it can choose to succeed instead.

Magic Savant. The owl can sense the presence of magic within 60 feet of it, and within 30 feet of its sensors. If the owl senses magic in this way it can use its bonus action to see a faint aura around any creatures or objects, within 60 feet of the owl or within 30 feet of its sensors, that bears magic, and the owl learns its school of magic, if any.

In addition, the owl can sense the presence of any creature that can cast at least one spell, within 120 feet of the owl or within 60 feet of one of its sensors. If the owl can see a creature, within 120 feet of the owl or within 60 feet of one of its sensors, it can tell if it can cast at least one spell. If the creature can cast at least one spell, the owl can learn what the creatures spellcasting ability is, whether it can cast the spells innately, how many spells it can cast, what schools of magic the spells are and which one is most common, what damage types it can deal with its spells and which one is most common, what the highest level the creature can cast a spell at, and what spells it can cast at-will (no action required). As a bonus action, the owl can focus on a creature that it knows is a spellcaster, and learns the following information, the number of spells the creature can cast of each spell school, of each level and of each damage type they can deal, how many levels the creature has in artificer, bard, cleric, druid, paladin, ranger, sorcerer, warlock, and wizard (for each class), what the creature's Constitution and spellcasting ability score is, what spells it can cast innately (this includes any non-at-will spell that it doesn't need to expend a spell slot to cast) and how often it can cast each of them, and what spell it casts most frequently that it can't cast at will. As an action, the owl can focus on a creature that it knows is a spellcaster, and learns the following information (in addition to the information it would have gained from doing this as a bonus action), for how long the creature has been able to cast spells, how often the creature looses concentration on its spells, if it uses its spells more frequently in combat or out of combat, any subclasses the creature has, any feats that relate to the creature's ability to cast spells (e.g. magic initiate, shadow touched, war caster), any magic items the creature has that allows them to cast a spell, what spells the creature can cast, how often it casts each of them, for how long it has been able to cast each of them, and where the creature gets the spell from (whether the via a class, race, magic item, feature, or otherwise).

Favor of the Owl. If a creature spends at least 1 minute within 30 feet of the owl, the owl can bless the creature, giving it the following benefits. A creature can sense the presence of poison, poisonous creatures, and disease within 30 feet of them, and can identify the kind of poison, poisonous creature or disease in each case. A creature can also sense if an object or creature is magical and can identify what school of magic is effecting it. The creature can also see invisible creatures and objects, within 30 feet of them, as if they were visible. The creature also automatically detects illusions and succeeds on saving throws against them, and can perceive the original form of a shapechanger or a creature that is transformed by magic. These effects last until the creature finishes a long rest (a creature could potentially gain the benefit of a long rest without having finished one).

Owl's Sight. As a bonus action, the owl can create an invisible magical sensor in a location the owl has seen within 1 mile of it, that hovers in the air. The owl receives visual and audio information from the sensor, which has darkvision out to a range of 120 feet and truesight out to a range of 60 feet. The sensor can look in any direction. The sensor can also see through solid objects out to a range of 30 feet, but 2 feet of rock, 2 inches of any metal other than lead, or a thin sheet of lead blocks its sight. The owl can have a total of 10 of these sensors active at a time, but only 5 of them can persist outside of the owl's lair. 

The owl can move each sensor 30 feet in any direction on its turn — no action required, or 60 feet in any direction as a bonus action on its turn. The owl can also lock an sensor onto a creature as a bonus action, if the sensor is within 15 feet of the creature. While an sensor is locked onto a creature the sensor moves with the creature, staying within 15 feet of the creature, until the owl unlocks the sensor (no action required) or locks the sensor onto a different creature — the owl can still move the sensor but it must remain within 15 feet of the creature. There is no limit to how far away from the owl the sensor can move, but it must stay on the same plane of existence as the owl. However, the sensor can move through a solid barrier, but 2 feet of rock, 2 inches of any metal other than lead, or a thin sheet of lead blocks its movement, and it can't end its movement while in a solid object.

When the owl uses its action to cast a spell, the owl can cast the spell through a sensor as if it was there. When it does so, if the owl can cast the spell at-will, any creature that can't see the sensor has disadvantage on the saving throw.

A creature that has a passive Wisdom (Perception) of at least 22 and can see invisible objects (such as a creature benefiting from see invisibility or truesight) sees a small, luminous, intangible flying owl.

Speak with Beasts and Plants. The owl can communicate with beasts and plants as if they shared a language.

Supreme Foresight. The owl gains a bonus to its initiative rolls equal to its Intelligence modifier. It also gains a bonus to all of its saving throws equal to half of its Wisdom modifier (not included in the saving throw bonuses), and if the owl is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails. Attack rolls made with advantage against the owl aren't affected by advantage. If a creature scores a critical hit against the owl, and it doesn't have advantage on the attack roll, the creature doesn't roll the damage dice twice.

In addition, when a creature misses the owl with an attack, the owl can cause the attack to target a different creature, other than the attacker, within 5 feet of the owl (using the same attack roll), or the owl can use its reaction to cause that attack to hit one creature of its choice within 10 feet of it.

Telepathic Bond. If the owl communicates with a creature telepathically, the creature can respond to the owl, and other creatures in the conversation, likewise.

Titan's Stability. The owl can only become prone if it wishes to be.

Will of the Ancient One. If a creature spends at least 1 minute (10 turns) within 30 feet of the owl, and gains the benefit from the owl's Favor of the owl or directly benefits from one of the owl's spells, it it gains the mark of the owl. The mark of the owl lasts on a creature for a number of days equal to the number of hours (rounded up) the creature spent within 30 feet of the owl. If a creature with the mark of the owl targets a good creature with an attack or a harmful spell or includes it in the area of effect of a harmful spell, it must first make a DC 27 Wisdom saving throw. On a failed save, the creature must choose a new target or take a different action. A creature with the mark of the owl gains the following bond: "My heart and mind must stay forever clear; I cannot just act on a whim or fancy, everything must be carefully deliberated."

In addition, a creature with the mark of the owl has advantage on Intelligence (Arcana) checks, Wisdom (Animal Handling) checks, and Charisma (Persuasion) checks — made to persuade a good creature. A creature with the mark of the owl has truesight out to a range of 60 feet, and has advantage on saving throws against spells and other magical effects.

Multiattack. The owl casts one of its at-will spells and makes a talons attack.

Talons. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 19 (4d6 + 5) slashing damage plus 20 (8d4) force damage, and the target is grappled (escape DC 21). Until this grapple ends, the target is restrained, and the owl can't use its talons on another target. If the owl teleports while it has a creature grappled, the creature teleports with it.

The owl 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 owl regains spent legendary actions at the start of its turn.

Move. The owl moves up to its speed without provoking opportunity attacks.

Spell. The owl casts one of its spells.

Talons Attack. The owl makes a talons attack.

Spells (Costs 2 Actions). The owl casts two of its spells. If either of them target a single creature the owl can target two creatures with it.

Swoop (Costs 2 Actions). The owl moves up to half its speed, without provoking opportunity attacks, and makes a talons attack, then the owl can move up to half its movement speed, without provoking opportunity attacks.