Moon is a 16 year old girl. Leader of all terraia. But she never knew of the mysterious moon."glow can you come here"said moon. "Yes moon"said glow  "I've been hearing of a mysterious moon I was wondering if you have any information about it."said moon "I can, the mysterious moon a special red moon mood that happens every 20 days"said glow. "The next moon will happen in two days." "Glow   could you and Zarina set up a nice place where  the three of us may observe this red moon"moon asked "yes we can do that but why us why do Zarina and me get to do things with you all the time" asked glow. "You and Zarina are my most trustworthy servants and I can always count on you."said moon "okay I'll get Zarina and we'll get working right away." Said glow. "Thank you." Moon glances at a picture of the wall of flesh "you'll always will be in my heart wof." 

2 days later 

"Its becoming night" said moon "there it is the red moon." Said Zarina. "Wow it's the coler of blood, bright red" said moon "there sure is a lot of activity tonight"said Zarina "the moon makes them swarm"said glow "I shall call it the blood moon an event where all mobs shall swarm the players of that world" said moon 

that is how the blood moon came to be.