Blue Dragon Wyrmling (Boss)

Flyby. The Blue Dragon Wyrmling doesn't provoke opportunity attacks when it flies out of an enemies reach.

Tunneler.  The Blue Dragon Wyrmling can burrow through solid rock at half its burrowing speed and leaves a 5 foot-wide, 5-foot-high tunnel in its wake.

Lightning Absorption. Whenever the Dragon is subjected to Lighting damage, it takes no damage and instead regains a number of hit points equal to the Lightning damage dealt.

Charge UP! When the Dragon reaches half health it immediately recharges its Lighting Breath attack.

Multiattack. The Blue Dragon Wyrmling makes two Claw attacks and one Bite attack.

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (2d4 + 3) slashing damage. 

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (1d10 + 3) piercing damage plus 3 (1d6) lighting damage.

Lightning Breath (Recharge 5-6). The Dragon exhales lighting in a 30-foot line that is 5 feet wide. Each creature in that line must make a DC 12 Dexterity saving throw, taking 22 (4d10) lighting damage on a failed save, or half as much damage on a successful one.

Summon Minions. The Dragon calls on a number of Kobolds it allows to inhabit its cave for help. The Dragon calls upon (1d4+2) within 30-feet in an unoccupied space of it.

Tail Lash. The Dragon senses movement behind it and lashes out with its tail. Whenever a player character moves  within 5-feet of the Dragon, it may make a free reaction, doing a standard 7 (1d8 +3) bludgeoning damage and knocking the player character back 10 ft. In a straight line, the player must also make a DC 12 Dexterity saving throw or be knocked prone.