Assassin Beast

Quick Evolution. At the start of its turn, the assassin beast can choose one of the following damage types: acid, poison, cold, fire, necrotic, or lightning. It gains resistance to that damage type. It can also choose one of the following effects to apply to itself. It doesn't need to choose a different one each time, and it can choose none if it wishes. If it chooses a damage type or effect different from the one it already has, the previous damage type or effect is removed, and the new one takes its place.

• Magic Resistance. The assassin beast has advantage on saving throws against spells and other magical effects.
• Evasion. When the assassin beast is subjected to an effect that allows it to make a Dexterity 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.
• Prediction. Melee attacks against the assassin beast have disadvantage.
• Spider Climb. The assassin beast can move up, down, and across vertical surfaces and upside down along ceilings, while leaving its hands free.

Shapechanger. The assassin beast can use its action to polymorph into a Small, Medium, or Large humanoid it has seen, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Multiattack. The assassin beast makes a number of claw attacks equal to the number of arms its current form has (maximum of four), or four if in its true form. It then makes a tail attack.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 21 (4d8 + 3) slashing damage. 

Tail (True Form Only). Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 14 (3d6 + 4) piercing damage and 10 (3d6) damage, choosing from acid, poison, cold, fire, necrotic, or lightning.

Detect Vulnerability. The assassin beast peers into the mind of a creature it can see, and that creature must make a DC 16 Wisdom saving throw, with disadvantage if they are unaware of the assassin beast's presence. On a failed save, the assassin beast learns that creature's saving throw proficiencies, any damage resistances or immunities it has, any condition immunities it has, and its challenge rating or level. If the creature has multiple classes, it only learns their total level.

Adapt Combat. The assassin beast focuses on a creature it can see within 30 feet, assessing its fighting style and attack patterns. Until the start of the assassin beast's next turn, that creature has disadvantage on attacks against it and the assassin beast has advantage on attacks against them.

Predictive Strike. When a hostile creature enters within 5 feet of the assassin beast, it can use its reaction to make a claw attack against them.