Big Bill of Big Bill Hells

Planar Shift/Domain. Upon reaching 50% max HP, Big Bill transports him and his opponents into a pocket hell dimension

Spellcasting. Big Bill is a 6th-level spellcaster. His spellcasting ability is [5] (spell save DC 15, +5 to hit with spell attacks). Big Bill has following Sorcerer spells prepared:
Cantrips (at will): Control Flames, Fire Bolt, Green Flame Blade, Blade Ward, Frostbite
1st level (4 slots): Burning Hands, Absorb Elements, Grease, Chaos Bolt                              2nd level (3 slots): Scorching Ray, Flaming Sphere, Flame Blade                                            3rd level (3 slots): Fireball, Flame Stride, Melf's Minute Meteors 

Brass Knuckles. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit:2 (1d6 + 6) damage. 

Glock 19. Ranged Weapon Attack: +4 to hit, range 30/60 ft., 1target. Hit: 1 (1d8 + 4) damage. Opponent must make a DC 12 dex check or automatically be critically hit (15 round before needing to reload)

Reload. Bonus Action, big bill reloads his gun and regains all ammunition

Counter. Reaction, upon getting hit with a melee attack, Big Bill can roll an attack action to counter and deal 1d4+6 damage to his attacker

Big Bill Supernova (Costs All Actions). Big Bill creates a condensed sun in his hands and throws it at the ground, all opponents and allies must make a DC 15 constitution saving throw or be instantly knocked unconscious, on success they remain at 1 HP (Big Bill is rendered Defeated after using this attack, making it his desperation move, and not one to simply use freely)