Strongest Rings in Demonic:
S: Bloodied, Victorious, Prophet, Depth
A: Sanctified, Mystic, Lava, Heir, Archmage
B: Brightening, Farseer, Destiny, Astral, Charged, Love (early Riordan)
C: Tranquility, Harmony, Calamity, Companion
Bloodied is good on anyone; however, unlike other difficulties, it should be placed on a unit strategically to avoid doubles or to double.
Destiny gives 40 crit when waiting until your next attack. This stacks. Really good on buff bots and Reapers.
Love is great on Riordan as an extra 20 hp as early as Chapter 6 can make a huge improvement on any class he goes into.
Calamity nice little QoL boost. Another trick is you can do is to make healing effects proc from things like Haorun's passive, Prophet/Bright/Lifebinder Rings on units that have them. (Ex: Haorun has Calamity Ring and Prophet = he will proc his passive and Prophet Ring on himself when someone dies on map.)