Arcane Assassin

Arcane Weaponry. The assassin's weapon attacks are magical for the purposes of overcoming resistances and immunities to non-magical attacks and deal an additional 14 (4d6) force damage (included in the weapon's attacks). 

Assassinate. During its first turn, the assassin has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the assassin scores against a surprised creature is a critical hit.

Evasion. If the assassin is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the assassin instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Infiltrator's Spellcasting. The assassin can cast the following spells innately, requiring no material components. Its spellcasting ability is Intelligence (spell DC 16, +8 to hit with spell attacks).

At will: Charm Person, Disguise Self, Friends, Jump, Mage Armor, Mage Hand, Message, Sleep

3/day: Alter Self, Invisibility, Knock, See Invisibility, Silence

1/day: Dispel Magic, Misty Step, Nondetection, Passwall

Magic Resistance. The assassin has advantage on saving throws versus spells and other magical effects.

Perfect Infiltration (once per long rest). As an action, the assassin casts Plane Shift or Teleport. Upon arrival at the destination, the assassin, all creatures that were teleported with it, and any objects being worn or carried by them immediately become invisible. The invisibility on each creature lasts for one hour as long as the creature maintains concentration (as if concentrating on a spell) or until it attacks or casts a spell. 

Sneak Attack. Once per turn, the assassin deals an extra 28 (8d6) 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.

Multiattack. The assassin makes two shortsword attacks.

Shortsword. Melee Weapon Attack: +10; to hit, reach 5 ft., one target. Hit: 7 (1d6 + 5) piercing damage plus 14 (4d6) force damage.

Light Crossbow. Ranged Weapon Attack: +10; to hit, range 80/320 ft., one target. Hit: 8 (1d8 + 5) piercing damage plus 14 (4d6) force damage.

Phase (once per short or long rest). Until the end of its next turn, the assassin enters a spectral form. While in this form, the assassin is vulnerable to force damage, regardless of any resistances it would have normally to that damage, and it can move through objects as if they were difficult terrain. The assassin takes 5 (1d10) force damage if it ends its turn inside an object. If the assassin is within an object when the ability ends, it is shunted into the nearest unoccupied space, taking an additional 5 (1d10) force damage for every 5 ft it was shunted.