I Become the Demon King by Taming Slimes

"One soul from another world, a miserable start, and a special halo made for the protagonist. Just like that, you've got a hero."

"Wait, why is this hero's halo emitting black smoke?"

--

Ryan transmigrates to another world where extraordinary [Classes] exist, becoming a low-level adventurer stripped of all his gear and nearly starving to death in a labyrinth from the start.

Fortunately, he obtains the Demon King's Crown and fuses with a ghoul's projection. He begins contracting demonkin followers, and from then on the labyrinth has its strictest lord.

[Ghoul] fusion successful. Abilities acquired: [Darkvision] [Blood Scent] [Shadow Manipulation] [Minor Regeneration].

[Ghoul] evolves into [Vampire], then [Bloodline]... and evolves into [Demon King].

Contracted demonkin follower [Slime] evolves into [Devouring Slime]... then [Leviathan Slime].

Contracted demonkin follower [Demon Hound] evolves into [Bloodhound]... then [Hellhound].

And so Ryan begins hunting adventurers in the labyrinth, raising followers, and gaining extraordinary powers.

After Ryan conquers every labyrinth and opens the Demon King's palace, he finds a strange figure sitting on the throne, covered in black smoke and sporting horns, shouting something like, "Demons, your emperor has returned!"

Ryan slaps him into the wall. "Who are you, and what are you doing in my house?"