Secret world seeds - Official Terraria Wiki

Secret world seeds are world seeds which have been discovered as secret easter eggs that generate worlds with strange or unique features. Unlike regular world seeds – which only affect the usual world structure – secret seeds can produce worlds with characteristics that would not be attainable through normal world generation (i.e., they can affect the game's mechanics while playing in that world). For instance, they may change enemy statistics and behavior, item drops, graphics, theme music, and more.

Secret world seeds - Terraria Wiki

World - Official Terraria Wiki

GuideNPC Happiness - The Official Terraria Wiki, PDF

Secret world seeds - Official Terraria Wiki

World Seed - Official Terraria Wiki

Best secret world seeds for Terraria 1.4

Glitched World? : r/Terraria

Every NEW SECRET Seed in Terraria 1.4.4 Labor of Love

World - Terraria Wiki

No Traps - Terraria Wiki

Working as Designed - World Generation for Secret Seeds

Terraria: Secret Seeds For Unusual Worlds

Don't dig up - Official Terraria Wiki

I found a secret seed what does this do : r/Terraria