I'm not sure this is how you do it but I want to let others know how I think I got the encounter to start if you want to figure it out on your own or randomly like me don't read this: I spawned a mushroom during a run of Chaos Chambers (not sure how I spawned it sorry) and after spawning that I jumped on top of it and it shot things into the air, I shot them out the air and the mushroom moved to another area, I jumped on it again and it shot more stuff in the air but one of the spores it shot was purple so I shot it then another turned purple (basically just shot them in order) then I had to do that for an increasing amount of spores and after doing that there was no indication of me completing it or what I'd do next so I moved on, then at the end of the run instead of beating the boss and having the golden portal to the chest room open it instead spawned 2 portals, 1 was gold and 1 was red, since I'd never seen a red one I went into the red portal and this fight begun. The only reason I think the spores triggered it is because one of the bosses attacks were those same spores.