Is this Half-dragon Quaggoth boss monster balanced?

Question

This is going to be an early boss for my upcoming campaign. I have 7 5th-level player characters, and this fight is meant to challenge them quite a bit. There will also be a few minions.

The story is that this quaggoth found a dying dryad and consumed her heart, this made him a sorcerer as well as giving him some druid magic. Then the cult of the dragon found him and made him a half dragon. If the players don't kill him I plan on making him an NPC later on.

I would like to know if this monster is ready to be thrown at my players, if it is too overpowered or underpowered at its current challenge rating, and what I can do to fix any balancing problems found in the stats of this monster.

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Treill

Half-dragon Quaggoth Thonot (4th-level sorcerer)

Armor Class: 16 (permanent Barkskin)

Hit points: 48

Str (16; +3) Dex (12; +1) Con (16; +3) Int (8; -1) Wis (12; +1) Cha (14; +2)

Saving Throws: Str +6, Con +6

Senses: Treill has blindsight out to 10 ft, and darkvision out to 180 ft.

Challenge: 7

Wounded Fury. While he has 10 hit points or fewer, Treill has advantage on attack rolls. In addition, he deals an extra 7 (2d6) damage to any target he hits with a melee attack.

Lightning resistance.

Multiattack. The quaggoth makes two claw attacks.

Claw Attack. Melee Weapon Attack: + 5 to hit, reach 5 ft., 1 target. Hit: 5 (1d6 + 3) slashing damage.

Lightning breath Recharge (5-6). Treill can use his action to exhale destructive energy. Each creature in a in a 5 by 30 ft. line must make a Dexterity saving throw or take 24 (7d6) lightning damage, or half as much on a successful save.

Spellcasting. Treill is a 4th-level caster; his casting ability is Charisma. Spell save DC 13; +5 to hit with spell attacks.

Cantrips (at will) fire bolt, sword burst, poison spray, predestination, toll the dead.

1st level (3 slots): color spray, magic missile, shield.

2nd level (2 slots): invisibility, spider climb.

Innate spellcasting. Thonot DC 12. +4 to hit.

At will: mage hand, feather fall.

1/day: Cure wounds, enlarge/reduce, heat metal, mirror image.

Magic initiate (druid). DC 12, +4 to hit with spell attacks.

Cantrips: Thorn whip, druidcraft

1/day: healing word

Answer 1

This would not be a real challenge for 3 characters

Your creature is about CR2. 3 moderately optimizer characters would rip it apart in 2 rounds. Against 7, it should not last a round.

How it should look

You say you want to challenge your players, this sounds like at least a Hard encounter, 5250 XP for 7 players. For a single creature it is CR9.

DMG says (page 274), such a creature should have AC 16, attack bonus 7, HP 191-205, damage 57-62.
Your's is very far behind everything but AC. On damage and HP it is downright crippled, a CR1 is supposed to have more HP.

Answer 2

No, this monster is much too weak for CR7.

The guidelines tell us that a CR7 monster is expected to have AC 15 and 161-175hp, with 45-50 damage per round (averaged over three rounds) at +6 to hit (or DC15 save). Of course individual monsters will vary, but your monster has far too few hit points and also does not deal enough damage.

There's a table in the DMG (p. 274) with this information.