The [b]Channeling/Flowing/Crushing[/b] sets are the [i]primary[/i] Armor sets that drop from Prophecy while the [b]Moonfang[/b] Armor sets are a special, [i]secondary[/i] set with specific rules that govern how pieces are obtained.
The [b]Moonfang[/b] set is on a weekly, Class-specific knockout system, meaning the first time you complete the Dungeon with a given Class per week you will receive one piece from their respective set which you have not yet unlocked; subsequent runs during that week with the same Class will result in no further [b]Moonfang[/b] set drops, however you [i]can[/i] get one drop for each Class within the space of one week. As such it will take a minimum of five completions over five weeks to complete the full set for a given Class.
Luckily the [b]Moonfang[/b] set pieces are granted [u]in addition[/u] to whatever other drops one receives from the final boss Chest rather than replacing any single drop one would otherwise receive.
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That’s called rng, but also I don’t know why you would need or even question more armor drops. The armor doesn’t do anything for you and the guns are what you should be grinding for.
And btw 3 runs isn’t enough to get you what you want most of the time.