Yep, it happens after beating the bosses.

Ah, very cool then. Didn't know that. Thanks man.

Best stay alert in black forest at night as well :)

All things considered, mountains are probably the least hostile biome after bosses are killed.

And yeah, definitely don't have a friend start the game by joining your world where yagluth has been beaten...

ohhhhhhhhhhh......... ok...... that makes a ton of sense now.... i made a 2nd char and world to play around and test stuff. didnt build anything but i went godmode and flew around and tried out the plains boss when i was bored one day. welp i decided to try that world hop method for gathering some wood real fast, and used that world. i was sooooo confused just now why i just got ran down by some goblinslayer goblins 3 deep in my ♥♥♥♥♥♥♥. i had just farmed enough iron to make full iron armor, and then took it to lvl 4 all in one go. and they still shreked my ass. will they start spawning on my world? or only on the one i killed yag on?