Want to start Infernum mode but... (read text)

I want to begin an infernum mode run, but the most Calamity i have played was up to a half arsed Astrum Areus attempt in Revengance mode. So i know im in for a treat. Im playing with a mate so at least i have someone to beat my head against he wall with.

What should i expect, what changes does it make and how does it differ from base Calamity?

I have heard that its more consistent when it comes to RNG but asside from that i have no clue what it changes.

Thanks! -Certified Looser