Weaver

Shapechanger. The Weaver can use its action to polymorph into a Large or smaller creature 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.

Spellcasting. The Weaver is an unknown-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). The Weaver has every non-Cleric or Paladin spell prepared, and has 150 mana. It can't cast 8th or 9th level spells. This limitation is lifted if its INT changes to 15 and 17 respectively (usually magical). Mana is fully regained after a long rest as long as it is 10 Hours long (10 % less mana regained for every hour lost). If INT is increased, it gains 25 mana for every point gained. Each spell costs its  level squared amount of mana as described below:
Cantrips (at will)
1st level (1 mana)
2nd level (4 mana)
3rd level (9 mana)
4th level (16 mana)
5th level (25 mana)
6th level (36 mana)
7th level (49 mana)
8th level (64 mana) [Unuseable until 15 INT]
9th level (81 mana) [Unuseable until 17 INT]

Keter. Its actual CR is 40.

Knoledge knowing. A Weaver can sense beings with 12 or higher INT in 60 ft radius around it.

Bound by form. The weaver can't cast spells while not in its original form.

Regenerate. The weaver regenerates 1 hp every turn.

Instinct. Its INT is 15 mechanically but is actually unintelligent. If magic or anything else improves it and isn't static (such as changes to 19 unless higher), instinct is raised as well (for example, INT of 5 is 17 instinct).

Multicast. The Weaver casts a spell twice a turn (+10 * level/2 mana. For example, a 5th and a 4th level spell cost +45 mana more (20+25)).

Grapple. Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Anyone hit by the attack is grappled (Hit instead of DC, because it can't grapple when holding armor). 

Piercing Crush. The Weaver uses grapple, and then turns its body spiky. It then uses the following attack: Melee Weapon Attack: +15 to hit, reach 5 ft., 1 target. Hit: 20 (4d6 + 6) piercing damage. 

Excrete Poison. After using Piercing crush, it can then can use poison to make the opponent poisoned. When used, the target must make a DC 25 Constitution save or be poisoned for 1d6+1 days. 

Overtake. Anyone poisoned by excrete poison must make a DC 20 STR or WIS (your choice) save every turn or they will be controlled by the weaver for the turn (phisically, not by charming). The affected target can make a death saving throw insted and forgo acting. Once all 3 saves succeed, the target is cured, but on a faliure, the target remains concious and sentient, but becomes a weaver with only its eqipment and intelligence (as instinct) remaining. When a weaver sees a humanoid, it must make a DC 10 WIS or CON save (wichever is lower) or attack that humanoid until it is also a weaver.

Enter the description for how legendary actions work for your monster here.

Terror (Recharge 6). All beings in 60 ft. aware of the Weaver makes a DC 21 WIS save or be frightened for a minute.