In Eternity Warriors III, to get an understanding of the different tiers of items you just have to remember a couple key information.
There are 3 main bosses. [ Boss Name (Location) Tier / Reagent Needed for upgrades
1. Cyclops (Cyclops Chamber) Tier 1 / Minions
2. Nian (Hall of the Devine) Tier 2 / Fiends
3. Netherborn (The Ivar Mountains) Tier 3/ Demons