With the contributions/suggestions of @
cbwjm and @
Kalmarjan and taking into consideration @
Haran Yakir party he wants to take on. FYI, an odd thing with spell casting monsters in 5e is that "spell casting" is not listed in their actions, only their traits. Anyway, here is a revised and slightly more powerful version of the spellweaver (PS. I actually used the spell table from 2e to roll most of these spells):
EDIT: I corrected to the CR and XP to reflect the spells selected
Spellweaver
Medium humanoid, unaligned
Armor Class 14 (17 with mage armor)
Hit Points 96 (16d8 + 24)
Speed 30 feet.
STR 10 (-), DEX 18 (+4), CON 16 (+3), INT 20 (+5), WIS 18 (+4), CHA 14 (+2)
Saving Throws Dex +8, Int +9, Wis +8
Skills Arcana +13, Perception +8
Damage Resistances psychic; bludgeoning, piercing, and slashing that is nonmagical
Condition Immunities charmed, frightened
Senses truesight 100 ft., passive Perception 18
Languages -, telepathy 120 ft., see also telepathic bond
Challenge 10 (5,900 XP)
Chromatic Disk. Spellweavers carry a 6-inch disk of an unknown and nearly indestructible material. The disk stores magical energy, containing up to 10 sorcery points that the spellweaver can use as a bonus action. Once the spell points are used, they are gone. However, the disk can be recharged by absorbing magic. The disk gains one sorcery point for each level of the spell it absorbs, up to its sorcery point maximum of 10. Refer to magic resistance.
Innate Spellcasting. The spellweaver’s spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). The spellweaver can innately cast the following spells, requiring no material components.
At will: detect magic, invisibility
1/day: planeshift (self only)
Magic Resistance. The spellweaver has advantage on all saving throws against spells. In addition, if the spellweaver succeeds on its saving throw it may use its reaction to capture the spell in its chromatic disk. The spellweaver suffers no damage or effect from a spell it has absorbed with its chromatic disk.
Metamagic. The spellweaver has 12 sorcery points and knows the distant spell and empowered spell metamagic abilities. It can convert spell slots and sorcery points in the same manner as a sorcerer.
Shielded Mind. Attempts to communicate telepathically with the spellweaver, or to read its mind, always fail. A creature making an attempt must succeed on a DC 14 Wisdom saving throw or be affected by a confusion spell.
Spellcasting. The spellweaver is a 12th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). The typical spellweaver knows the following sorcerer spells prepared:
Cantrips (at will): acid splash, chill touch, mage hand, minor illusion, prestidigitation, shocking grasp
1st level (4 slots): charm person, feather fall, mage armor, magic missile
2nd level (3 slots): blur, darkness, levitate
3rd level (3 slots): dispel magic, fireball, slow
4th level (3 slots): confusion, blight, polymorph
5th level (2 slots): cloudkill, wall of stone
6th level (1 slots): chain lightning
Spellweaving. Each spell level cast takes one hand. (A third level spell costs three hands, a fourth costs 4, etc.) The Spellweaver may cast as many spell levels as he has hands, in any combination. It must pay the slot cost for each spell cast, but may not exceed 6th level. Cantrips take up one hand. In addition, a spellweaver can concentrate on up to two spells at a time and only one spell is interrupted each time it fails a concentration check.
ACTIONS:
Multiattack. The spellweaver can make up to six slam attacks.
Slam. Melee weapon attack: +8 to hit, rech 5 ft., one target. Hit: 2 (1d4) bludgeoning damage.
REACTIONS:
War Casting. When a creature provokes an opportunity attack, the spellweaver may cast a spell with a casting time of 1 action and that targets only that creature, rather than making an opportunity attack. This casting does not count against its spellweaving total, but it does cost 1 sorcery point.
Vindictive Casting. When a creature within 120 feet of the spellweaver cast a spell, the spellweaver may cast a cantrip as a reaction. This casting does not count against its spellweaving total, but it does cost 1 sorcery point.