So before you actually enter the first boss, there is the selection where you have the wall of chains and behind them you find gas spores or portals, here is where we use our given demonic power (F) to break the chains to choose what we want and don't want during the first boss.
What is happening repeatedly now is every time all chains break even if no one is going near them and you need to fight both rooms with gas spores and both rooms with mobs, therefor you find both at the first boss when you finally do enter.
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