With the dawn of Episode: Heresy in Destiny 2 comes a plethora of new and perplexing items to be added to your inventory, including Nether-found resources like Metastasized Essentia. If you're anything like me, you pick up anything you can while exploring in games and heading into the Nether meant plenty of things for you and me to interact with. However, you can't use Metastasized Essentia as soon as you pick it up—you'll need to progress the seasonal quest first.
Metastasized Essentia is a round, blob-like resource that is new this season in Destiny 2. It's not super common but has a few uses that will likely be expanded upon as the season progresses. For now, you should collect as much of it as possible because even its current, albeit limited, uses are helpful in grabbing extra rewards and upgrades.
Enter, And Successfully Complete, A Nether Run

Metastasized Essentia can only be found in one place (so far) in Destiny 2: The Nether. No, not like the Minecraft one; although Destiny 2's Nether is just as unforgiving, it's much easier to reach. You'll be funneled into the Episode's unique activity pretty early on in the Episode questline, wherein you'll be tasked with exploring Oryx's Dreadnaught patrol zone, now called the Nether.
This open-ended activity can be done alone or with a fireteam of two other Guardians, and it doesn't seem like the difficulty scales, so your best bet for getting out alive is by grabbing some buddies before heading in. Once you arrive, you only need to look for resource pickups using your Ghost. Pull out your Ghost to activate the scan function, and look for the pyramid-shaped indicators on the HUD that point towards destination resources, like they would with Microphasmic Dattalatice on Nessus.
Until you complete the Nether activity, the Essentia in your inventory will be "Fleeting" and disappear if you wipe to orbit.
The trick with Metastasized Essentia is that just because you've picked it up doesn't mean it's yours. In order to successfully obtain the resource, you need to complete the Nether activity without using all your wipes and being returned to orbit. The wipe mechanic functions as it does in Legend Lost Sectors, where the team has a certain number of revive tokens that, when depleted, will cause a full reset of the encounter or activity. Unless you've picked up the upgrade that allows you to keep some of your resources even if you fail, which you can't do without at least some Metastasized Essentia, you'll lose everything you've gotten if you fail the activity.
How To Use Metastasized Essentia
Visit Eris' Apartment In The Last City
Once you've gotten a few Nether runs under your belt, you should get a quest step taking you to Eris' apartment, where she's left a few things behind. Firstly, you should mourn the loss of one of Destiny 2's most interesting NPCs, but then you should check out the other stuff in her home. The first is the Shaping Slab, which is this Episode's seasonal "vendor." Here, you can get rewards for vendor rank as well as pick up any engrams you earn in the Nether.

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The second thing to check out here is the interactable plinth thing, called the Taken Altar, on the left side of the room. Here, you deposit your Metastasized Essentia in exchange for Essence of Desire, which is a seasonal currency used to open special chests in the Nether. You can also use the Tome of Want, which allows you to unlock extra reward drops. Metastasized Essentia is pretty hard to come by in Destiny 2's newest activity, but it'll be essential to chasing the rolls you're looking for.

Released August 28, 2017
ESRB T For TEEN for Blood, Language, and Violence
Franchise Destiny