Event
Blood Moon
Blood Moon is a Terraria event. 18 NPCs show up during it: 
Blood Eel, 
Blood Squid, 
Blood Zombie and 15 more.
What spawns here
Enemy 18
Fishing
Advanced Combat Techniques 1 in 2 (Legendary catch roll)During a Blood Moon; Before Advanced Combat Techniques has been used in this world
Dread of the Red Sea 1 in 2 (Legendary catch roll)During a Blood Moon