Blackened Blades Summoner

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

Evasion. If the summoner 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.

Sneak Attack. Once per turn, the summoner deals an extra 14 (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 summoner that isn't incapacitated and the summoner doesn't have disadvantage on the attack roll.

Multiattack. The summoner makes three Abyssal Blade attacks. One of the attacks can be replaced by  Hand Crossbow attack.

Abyssal Blade. Melee Weapon Attack: +9 to hit, reach 5ft., one target. Hit: 8 (1d6 + 5) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) necrotic damage on a failed save, or half as much damage on a successful one.

Hand Crossbow. Ranged Weapon Attack: +9 to hit, range 30/120 ft., one target. Hit: 8 (1d6 + 5) piercing damage, and the target must make a DC 13 Constitution saving throw or be poisoned for 1 minute. If the saving throw fails by 5 or more, the target is also unconscious while poisoned in this way. The target regains consciousness if it takes damage or another creature takes an action to shake it.

Spellcasting. The summoner casts one of the following spells, requiring no material componenets and using Charisma as the spellcasting ability (spell save DC 13):

At will: darkness, cause fear

1/day each: blindness/deafness, counterspell, dissonant whispers

Summon Demons (1/Day). The summoner attempts to summon one demon from the chaos of the Abyss with a 50 percent chance of success. If the attempt fails, the summoner takes 5 (1d10) psychic) damage. Otherwise the summoned demon appears in an unoccupied space within 60 feet of its summoner, acts as an ally of its summoner, and can't summon other demons. It remains for 10 minutes, until it or its summoner dies, or until its summoner dismisses it as an action. 

Shadow Step. While in dim light or dakness, the summoner teleports, along with any equipment it is wearing or carrying, up to 60 feet to an unoccupied space it can see that is also in dim light or darkness. It then has advantage on the first melee attack it makes before the end of its turn.