You'll need to find the Sunless Cell fragments in Destiny 2 if you want to complete the Dyadic Perfection quest and earn the final intrinsic upgrade for the Barrow-Dyad SMG. The Sunless Cell strike arrived alongside act three, so first you'll have to progress the seasonal story all the way to act three to unlock it. You'll also need to complete the Dyadic Ascension quest for Barrow-Dyad's other intrinsic upgrades...
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All Sunless Cell fragment locations in Destiny 2
Source: PC Gamer (opens in a new tab)