So my guide NPC was killed because I thought I could summon WoF by drowning the NPC.
(I realised I did it the wrong way).
I made a new world and made a hellevator after watching the guide of beating the WoF boss, the fight wasn't as bad and it gave me the items I needed. I finally got hardmode, I went back to my main world (where I drowned the guide), and still had the items. Then I got confused, it seems I need to ask a question I didn't find online...
Does your difficulty save when going on your main world, but defeated WoF on another world?
(this might be confusing but you get the point)