Arcane Wyrmling

Arcane Hunger. When the wyrmling bites a target with the spellcasting feature they must make a Constitution saving throw equal to their own spellcasting DC or they lose the highest unused spell slot equal to or less than DC - d20 roll giving the wyrmling temp HP equal to 1d4 per spell slot level. If the target has no unused spell slots to lose, they take extra necrotic damage equal to number of used spell slots. If this damage would kill the target creature, their soul is consumed and a new arcane wyrmling is born at the start of their next turn. Creatures killed in this way may only be brought back to life by a Wish Spell.  

Spell Form. The wyrmling is immune to damage from spells, if targeted by such a spell the wyrmling heals for 1d8 per Spell Level. Also, has resistance to magic weapons. The wyrmling is also unaffected by areas of magic effects and gains a +2 to attack and damage rolls and AC while in the area of an ongoing magic effect.

Arcane Sense. The wyrmling always benefits from the effects of the detect magic spell. In addition the wyrmling will always target the nearest creature with the highest unused spell slot level, and avoids creatures with no spellcasting ability. 

Power Overwhelming. If the spell slot consumed by Arcane Hunger is 7th+ level, then the wyrmling immediately uses Arcane Torrent regardless of recharge, if it was a 9th level than the temp hp becomes a permanent increase to current and max hp. At the end of its turn if a wyrmling has at least 100 max hp it increases by one size category and the amount of max hp to increase in size raises by 50. An arcane wyrm adds 1d8 to its bite damage per size category above small. At large Arcane Torrent becomes 1d10 health lost and 1d12 at gargantuan. It's STR score increases by 5, CON by 1, INT by 2, WIS by 1, and CHA by 1 per size increase. The Hit Die increases to 1d10 at Large, and 1d12 at gargantuan, the count increases by 3 per size increase, also raising CR. Spell Form heals for 2d8 at medium, 3d10 at large, 4d10 at huge, and 5d12 at gargantuan.

Arcane Torrent. Recharge (5-6) The wyrmling releases a blast of pure arcane power at the cost of its own life force. It loses 1d8 + highest level consumed spell slot health and all creatures in a 10 ft. radius sphere must make a DC 16 Dexterity saving throw or take 3 times the health lost force damage. On success a creature takes half damage. Regardless the wyrmling gains the benefits of the disengage action. 

Bite. Melee Weapon Attack: +7 (+9) to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4 + 2) piercing damage.

Spell Thief. The wyrmling targets a creature within 30 ft. that is concentrating on a spell. The target must make a DC 16 Constitution saving throw or take psychic damage equal to the level of spell being concentrated on and immediately lose concentration. The wyrmling innately understands the nature of the spell and may immediately cast it as part of this action. On a success the target takes no damage, remains concentrating and the wyrmling cannot cast the spell. Constitution is the wyrmlings spellcasting ability score.