I took an assassin as the template NPC for Illaris and grafted on some magus and racial abilities. I maxed her hit-points since she would be facing off against six characters. I ended up creating a chase scene as well which you can fine here.
Medium humanoid (half-elf), chaotic evil
Armour Class 15 (studded leather)
Hit Points 144 (12d8 + 48)
Speed 30’ ft.
Imitative +5 with scimitar/wand
Str 12 (+1) Dex 16 (+3) Con 18 (+4)
Int 14 (+2) Wis 8 (-2) Cha 13 (+1)
Saving Throws Dex +7, lnt +5
Skills Acrobatics +7, Deception +4, Perception +2, Stealth + 11
Damage Resistances poison
Senses darkvision 60ft, passive Perception 12
Languages Thieves’ cant, Common, Elvish, Orc and Gothic
Challenge 8 (3,900 XP)
Arcane Pool. As a bonus action Illaris may temporarily convert her scimitar into a +2 magical weapon. This bonus is included in the weapons statistics.
Evasion. If Ilarris is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, Ilarris instead takes no damage if it succeeds on the saving throw and only half damage if it fails.
Fey Ancestry. Ilarris has advantage on saving throws against being charmed, and magic can’t put her to sleep.
Sneak Attack (1/Turn). Ilarris deals an extra 13 (4d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the assassin that isn’t incapacitated and the assassin doesn’t have disadvantage on the attack roll.
Spellstrike. Ilarris may cast any touch spell through her scimitar as part of her attack action delivering the spells full affect on a hit. She can do this only once per round and the spell is not wasted on a miss attack but is held up to 1 minute (only shocking grasp in this case).
Spellcasting. The Ilarris is a 4th -level spellcaster. Her spellcasting ability is Intelligence (spell save DC 14, +6 to hit with spell attacks). The Ilarris has the following wizard spells prepared:
Cantrips (at will): mage hand, shocking grasp (1d8, adv if metal), poison spray (R10’, CON, 1d12), ray of frost (1d8, -10’)
1st level (3 slots): chromatic orb (3d8 acid), magic missile (3x 1d4+1), shield, vanish (invisible for 4r)
2nd level (2 slots): glitterdust (2 targets, WIS, blinded), scorching ray (3x 2d6), mirror image (3 dupes, AC 13, d20 6+, 8+ and 11+)
Multiattack. Ilarris makes two scimitar attacks. Or one scimitar and one wand attack.
Scimitar. Melee Weapon Attack (magical): +9 to hit, reach 5 ft., one target.
Hit: 6 (1d6 + 6) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage on a failed save, or half as much damage on a successful one.
Morale Ilarris is vindictive but canny. If reduced to fewer than 30 hit points, she flees by using her rift boots or casting vanish. She’s hot-tempered and harbors grudges, so if she does escape, she immediately begins planning revenge.
Combat Gear wand of lightning bolt (20 charges); Other Gear thieves’ tools, spell component pouch, spellbook (contains all spells prepared, all cantrips, and acid arrow, alter self, burning hands, color spray, levitate, mount, silent image, spider climb, and true strike), 3 silverdisks, 48 gp