Imagine what happens when you step into the portal of the nether. 
Imagine a new overworld, but burned to crisp by the powerful sun and became netherack.

Imaginethe sun, melted the moon, stars and itelf.
Imagine the sun, the stars and the moon, created a red sky.
Imagine some netherack melted into lava.
Imagine the stars, became quartz and rained done into the netherack.
Imagine some stars, formed into Ghasts.
Imagine the Ghasts shot fire onto the land.
Imagine the pigs came in, and got infected into zombie pigman.
Imagine the skeletons came, and became Wither Skeletons.
Imagine all of them, build nether fortresses.
Imagine the lava, modeled the blazes.
Imagine the blazes, turned some fire into glowstone.
And Imagine the player, walked out of the portal, and begin his/her adventure.