The Calamity Mod is by far the most well-known mod in the Terraria community. It is one of the largest and best mods in TModLoader, adding hundreds of items and blocks, five biomes, a new class, over 30 unique songs, more than 50 recipes for previously uncraftable vanilla items, 23 bosses, and multiple hours of post-moonlord content. It is so well known, that many people consider Calamity's deep story to be a part of Terraria official lore.
Terraria Wiki