The drop rates on these items should be rare for RNG, but I believe if you complete a raid flawless you should be given the raid exotic.
If you have been able to master something flawlessly, there is no reason that can’t be rewarded.
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I think completing the raid should just award you an exotic quest thats steps require you to master the raid, doing a bunch of difficult triumphs and what not, maybe not petras run difficult but stuff like running the raid a certain number of times, running the raid with everyone on arc solar or void, completeing each weekly raid challenge for all of the encounters that have one and other stuff along those lines.