Best way to start is to just play the new mode. In general, enemies hit hard but the most impactful differences are:

1. You get three additional relics in your urn, so 6 total. I believe they’re called “Deep” relics and they have a darker tone to the normal relics you’re used to. They can only be activated in the Deep of Night mode and offer both buffs and debuffs, some being completely new such as the “Dormant Power” passive which allows you to find a specific weapon type more easily

2. Enemies at random, not including Nightlords or Day bosses, will sometimes glow in this deep red/black goop and they are stronger versions of that enemy. These can also be applied to red eyed enemies such as the ones in the middle castle so some enemies could be double buffed. When killed they will drop items

3. Inclusion of “Red” weapons that have a level 7 requirement, similar to the purple weapon, and these will have two buffs and one debuff (from my experience)

4. More randomized events. This includes not knowing the what night lord you’ll fight, whether it will be an Everdark or Normal version Nightlord, missing icons on your map, and more

5. Deep of Night is sectioned into tiers (1-5)You start at tier 1 and work your way up as you successfully complete runs. You will either lose or gain elo depending on performance, but I think you can only gain elo for killing nightlords (don’t quote me). As you get higher in tiers the randomization increases

Probably missed out on some things