Got to 5BC and what the heck is going on?

After one of my most weirdest run(getting flawless conjunctivitis outfit and beating giant with the blowgun) I finally managed to get the last bc. I just decided to do run to see how hard it looks and it’s just pure chaos.you can now get infected by malaise which I have no idea how it works with the tiers and random enemies coming out of ground.and most importantly why did collector got replaced with one of those green creatures?

Note: I would also appreciate some tips ‘n tricks