Ceaseless Void
So i've recently started playing calamity again after 2 years or so and this time I played expert with revengeance mode instead of just regular classic and ceaseless void was the first boss I really struggled with.
The dark energies are the main part of the fight I despise but not because of the dark energies themselves but because of them being locked to ceaseless void.
I do believe that the dark energies being locked to ceaseless void makes the fight pretty unreadable because whenever ceaseless void dashes towards you and the dark energies follow ceaseless void you really are your own enemy because its movement really just depends on how you move.
I dont have much experience with calamity and i've only beaten ceaseless void on classic mode and revengeance mode but none of the other ones however I believe that ceaseless void would actually be a pretty fun bullet hell boss if the dark energies wouldnt be locked to ceaseless void and instead just spawned where ceaseless void is. Feel free to prove me wrong tho.