Supreme calamitas has an anti-cheat mechanic. If it doesn't do enough damage to you for example it will instantly kill you. The wiki cites:
Supreme Calamitas has an anti-cheat mechanic that activates if the player either survives more than thirty hits over the course of a single fight or takes less than 20 damage in a single blow from any of Supreme Calamitas' attacks. The player instantly dies in the case of either scenario. This is to discourage the use of powerful equipment from other mods to trivialize the fight. With the
Flesh Totem equipped, the damage threshold is lowered to 10.
https://calamitymod.gamepedia.com/Supreme_Calamitas
I have some mods that have allowed me to stack up quite a bit of damage reduction and armor, and had just crafted a new set before calamitas, and apparently it had enough reduction so I took 1 damage from something and was instantly killed. Now I'm getting instantly killed without warning just from walking around in my castle.
even after removing my damage reduction items and reducing my armor, i still die in 1 hit, but there is no death message.
Edit: I'm pretty sure it's the calamity mod, because this has only started occurring after I summoned SC and no other mods impose any kind of instant-death mechanic that I'm aware of.
Edit2: overall I LOVE the calamity mod, I put 200 hours on it a few months ago with my significant other, but wanted to play the updated version. we started fresh, but with more mods (I upgraded my computer since then so we could handle more mods). everything went beautiful, no issues at all, until SC. I'd still like to try to fight, I've tried taking off all of my non-calamity and non-vanilla items, but i'm still just randomly dying...