I am not convinced that the 5e (based on the 3.5e) methodology for monster building is always the best course when it comes to spell casters. It can become overwhelming to run them due to all the spells. I tend to think the 4e method of building unique monsters when it comes to spell casters may actually work better. Here is an attempt at doing that with Beornre.
Medium humanoid (half-elf), neutral evil
Armour Class 15 (mage armour)
Hit Points 45 (10d8)
Speed 30 ft and Hex Jump.
|9 (-1)||14(+2)||10(+0)||17(+3)||13 (+1)||12 (+1)|
Skills Cybertheurgy +6, Arcana +6, Deception +5, Perception +4, Stealth +5
Senses darkvision 60 ft., passive Perception 14
Challenge 7 (2,900 XP)
Fey Ancestry. Beornre has advantage on saving throws against being charmed, and magic can’t put the drow to sleep.
Nanite Jump (1/Day). As a move action Beornre either teleports 60 feet or swaps positions with one hexed creature within 60 feet of him.
Nanite Surge (1/Day). As a bonus action Beornre’s body is infused with nanites. Once per day as a bonus action or a reaction, Beornre can cause his nanites to surge, granting a proficiency die bonus of +1d6 to any one d20; this ability must be activated before the roll is made. When Beornre uses this power, his eyes glow with red light equivalent to that of a torch in illumination for 1 round.
Nanite Strike. Melee Weapon Attack: +2 to hit, reach 5 ft ., one target.
Hit: 2 (1d6 -1) slashing damage, plus 3 (1d6) poison damage.
Baleful Wind. The target is washed in a sea of nanites that begin reshaping the creature into something… else. The target creature within 120 ft. that fails a DC 14 Constitution save is hexed and turned into a sheep (or similar tiny or small beast). While in this form, the target cannot use powers. Save ends.
Weaponised Meme. As a bonus action Beornre speaks an idea that no mortal was ever meant to hear let alone understand. All creatures that can hear the meme must make a DC 14 Intelligence save or become stunned for 1 minute as they try and “unlearn” what they just heard. If the target is attacked they automatically regain their senses but have disadvantage on all attacks against Beornre for the remainder of the encounter until they succeed at DC 14 Intelligence save.
Capricious Earth (1/Day). All creatures within melee (immediate) range take 8 (1d10 +3) bludgeoning damage. Furthermore creatures that fail a DC 14 Strength or Dexterity saving throw are pushed back 30 feet and the target is knocked prone.
Digital Devil (1/Day). Beornre magically attempts to construct a creature from the nanites in his body that is identical to a shadow demon mechanical. However Beornre only has a 50 percent chance of success. 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 Beonre dies, or until Beonre dismisses it as an action.
Shield. As a reaction Beornre can raise a magical shield cancelling any magic missile attack and adding +4 to his AC for that attack.
Beornre is Akradenn’s Number One. Beornre is a nano-mage and his magical abilities are all linked to his ability to manipulate matter using nanites that live in his body. Beornre can form a weapon from the nanites that live in his body (nanite strike) which form into a silvered blade for the strike and then disappears collapsing back into his hand. They also infect the target creatures and cause a poison like damage for a short time.