Hard Boiled Heavies

"A powerful robot army built by Dr. Eggman. Loyal to his orders, the Heavies successfully retrieved the mysterious gemstone, but its power seem to have loosened a few of their screws."

- Sonic Mania's description manual.

The Hard Boiled Heavies are a group of Egg-Robos that have been transformed into more powerful badniks. They were sent to dig up the Phantom Ruby, but because of it's energy wave, the team gets transported to Green Hill Zone. Because of this change, every robot has new personalities and characters with Heavy King as their leader.

Sonic Mania
In Sonic Mania and before their transformation, the five were first known as simple Egg-Robos that are tasked to retrieve the Phantom Ruby to their former leader, Dr. Robotnik. By doing so, the badniks dig up the jewel with a large claw who picks a large chunk from the ground of it. The gem is plunged but it sends out it's strange power, causing it to transport the five from Angel Island to Green Hill Zone. The team turns into twisted versions of themselves, all having a new change in their looks, personality and purpose. They run off.

Still in Green Hill, the Hard-Boiled Heavies stand upon Robotnik, who demands them to return the Phantom Ruby. Robotnik snatches it and the bots leave.

The badniks fight Sonic, Tails and Knuckles throughout the story, even trying to steal the Master Emerald from the echidna, which ultimately failed. But later in Titanic Monarch Zone, the squad of the Hard Boiled Heavies have now turned into the Phantom Heavies. A transformed version of already transformed bots that the three heroes would encounter during the fight with the Phantom Egg. Yet, they still were a blunder to Sonic, Tails and Knuckles' super states.

Sonic Mania Plus
In the Plus edition, the Hard Boiled Heavies (except Heavy Magician) have been defeated, left to rust somewhere on Angel Island. However, the Heavy Magician stole the Phantom Ruby and brings it to the Heavy King to revive the broken members.

The King uses it to teleport them to Green Hill once more and flee afterwards, fully restored. Robotnik angrily takes the ruby again from the team, also leading to them scattering apart.

After basically rewinding what happened in the other edition, Heavy King survives the destruction of Titanic Monarch Zone and attacks Sonic and his friends with a giant sundae when they were enjoying themselves at the ice cream parlor.

Sonic Mania Intro
While not making an appearance in the series, they do in the opening animation of the game. Heavy King menacingly looks at the trio before turning his head to Heavy Gunner, which is a sign of demanding him to attack Sonic.

In Studiopolis, Sonic calmly looks at the view and suddenly gets attacked by the Gunner. Heavy Gunner aims at Sonic, but Sonic unfortunately hears the sound of a weapon. He evades 3 blasts from Heavy Gunner before exploding him.

Trivia

 * Heavy Gunner is the only Act 1 Boss in the game out of the other Heavies, the others are bosses in Act 2.
 * The Heavy Gunner in the Studiopolis boss isn't able to damage. Only his companions are.
 * Heavy King is the only member to not use "Hi-Spec Robo Go!" as his theme. He instead uses "Havoc Prognosis".
 * Every member are bosses in both Sonic Mania and Sonic Mania Plus.
 * Christian Whitehead first imagined Dr. Robotnik in a depression after Sonic 3 & Knuckles. He'd decay sadly in his pyjamas whilst his abandoned Egg-Robos would go a "little mad".