Colossal Panther

Beast of the Shadows. The panther is unaffected by difficult terrain, and its movement speed cannot be reduced. While in darkness the panther is invisible to any creature that relies on darkvision to see in that darkness. It can be tracked neither by magical nor non-magical means, nor can it be targeted by any divination magic or perceived through magical scrying sensors, unless it wishes to be. In addition, each creature of the panther's choice, while within 40 feet of it, gains a +10 bonus to Dexterity (Stealth) checks and cannot be tracked by magical or non-magical means, nor can they be targeted by any divination magic or perceived through magical scrying sensors — unless the panther wishes them to be.

Evasion. When the panther is subjected to an effect, that allows it to make a Dexterity saving throw to take only half damage, it instead take no damage if it succeeds on the saving throw, and only half damage if it fails.

Forest Guardian. The panther gains a bonus to its initiative rolls equal to its wisdom modifier. In addition the panther cannot be surprised while it is conscious.

Innate Spellcasting. The panther’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: blur, darkness, enhance ability (cat’s grace, no concentration), ensnaring strike, find traps, freedom of movement, jump, locate animals or plants, longstrider, nondetection, primal savagery, silence, snare, spider climb, zephyr strike

3/day: wrath of nature (no concentration)

1/day each: hallucinatory terrain, locate creature

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

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

Pounce. If the Panther moves at least 20 feet straight toward a creature and then hits it with a claw attack on the same turn, that target must succeed on a DC 23 Strength saving throw or be knocked prone. If the target is prone, the panther can make one bite attack against it as a bonus action.

Shadow Step. When the panther is in dim light or darkness, as a bonus action, it can magically teleport up to 120 feet to an unoccupied space it can see that is also in dim light or darkness. The panther then gains advantage on its next melee attack it makes before the end of its turn.

Shadow's Strike. The panther deals an extra 21 (6d6) damage when it hits a target with a melee attack and has advantage on the attack roll.

Speak with Beasts. The panther can communicate with beasts as if they shared a language.

Superior Invisibility. As a bonus action, the panther turns invisible. Any equipment the panther is wearing or creatures it is willingly carrying become invisible with it. The panther is invisible to creatures even if they have truesight.

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

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

Transport via Plants. The panther can touch or step into into a tree or bush and teleport to another tree or bush it has seen before on the same plane of existence, by expending 5 feet of movement. 

Will of the Ancient One. If a creature spends at least 1 minute (10 turns) within 40 feet of the panther, and gains the benefit from the panther's Beast of the Shadows or has been a willing target of one of the panther's spells, it gains the mark of the panther. The mark of the panther 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 panther. If a creature, with the mark of the panther 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 panther gains the following bond: "Though most creatures do not need to know the truth, the world has enough liars in it already, so I will not join their number."

In addition, a creature with the mark of the panther has advantage on Dexterity (Stealth) checks, Wisdom (Animal Handling) checks, and Charisma (Persuasion) checks made to persuade a good creature. When a creature, with the mark of the panther, is subjected to an effect, that allows it to make a Dexterity saving throw to take only half damage, it instead take no damage if it succeeds on the saving throw. If a creature, with the mark of the panther, has the Evasion feature (or a feature that provides the same effect but under a different name), when the creature succeeds on a Dexterity saving throw it becomes invisible until the end of its next turn. A creature, with the mark of the panther, deals an extra 7 (2d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, if the target is evil.

Multiattack. The panther makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 18 (2d10 + 7) piercing damage.

Claw. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 16 (2d8 + 7) slashing damage.

Change Shape. The panther polymorphs into a cat, a panther, or back into its regular form. Its statistics, other than its size, speed, strength score, and attacks (bite and claw), are the same in each form. Any equipment it is wearing or carrying is absorbed or borne by the new form (the panther's choice).

Illusory Duplicate. The panther becomes invisible at the same time that an illusory double of the panther appears in its space — the panther can also have the illusory double take the appearance of a black cat or a black panther. The panther can move the illusory double — without it provoking opportunity attacks — when it moves or as a bonus action. The panther can also have it gesture, vocalize and behave in whatever manner it chooses (within the boundaries of what it could do normally) — no action required. The illusion is corporeal and holds up to physical inspection and it can carry and otherwise interact with creatures and objects as it could normally, however it can not deal damage or directly harm anyone.

The illusory double relays sensory signals to the panther — allowing it to see through the illusory double's eyes and hear through the illusory double's ears as if the panther were located where the illusory double is — without impeding the panther's own senses.

The illusory double uses the panther's stats when necessary. If the illusory double takes 10 damage from a single attack the illusory double disappears.

Shadowy Escape. The panther imposes disadvantage on an attack roll made against it, and if the attack hits, the panther halves the damage that it takes from the attack and becomes invisible until the start end of its next turn or until it deals damage to a creature.

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

At-Will Spell. The panther casts one of its at-will spells.

Claw Attack. The panther makes a claw attack.

Move. The panther moves up to its speed without provoking opportunity attacks. If the panther's illusory double is active, it can have it move up to the panther's speed, as well.

Illusory Duplicate (Costs 2 Actions). The panther uses its Illusory Duplicate.

Pounce (Costs 2 Actions). The panther moves up to its speed and makes a claw attack, if it knocks the target prone it can also make a bite attack.

Spell (Costs 2 Actions). The panther casts one of its spells.

Sleep (Costs 3 Actions). Each creature of the panther's choice, within 30 feet of the panther, must make a DC 27 Wisdom saving throw. On a failed save, the creature falls unconscious for 10 minutes. A creature awakens if it takes damage or if someone uses an action, while the panther is not within 30 feet of the creature, to shake or slap it awake.