Pandemonium

Pandemonium Is a hard mode event that has a 0,5% of happening during daytime, and it lasts until night time. Once the event happens, a message appears saying:

Satan is really mad at you...

Then all monsters will get replaced with underworld monsters, the eerie song will play, the background will become blood red and there will be black floating rocks seen in the background and all water will be replaced with lava until the event ends. There's also 5% chance to replace a normal monster spawn with the Hell Panda miniboss. Only one hell panda can be spawned at a time.