If you want to buy new blueprints and upgrade your gear in Nightingale, you'll need a lot of T1 Essence. Here are the best ways to get T1 Essence in Nightingale.
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The best place to get T1 Essence is in an Antiquarian Realm, which can only be accessed by using an Antiquarian Card on a Portal. Antiquarian Realms are home to more powerful enemies compared to Abeyance Realms; enemies will also spawn more frequently. For this reason, I recommend reaching at least Gear Level 20 before entering an Antiquarian Realm.
In an Antiquarian Realm, T1 Essence spawns the same way as Essence Dust. I highly recommend completing Occupations and Bastilles, which are more likely to drop T1 Essence than other encounters. You can also get T1 Essence by killing Bound, skinning animals, and looting chests, but these do not reliably drop T1 Essence. Hope Echoes in Antiquarian Realms consistently drop T1 Essence, but outside of Bastilles they are procedurally generated and difficult to find.
Essence Traders in Antiquarian Realms only accept T1 Essence as currency for most of their items. These traders sell crafting recipes and items that cannot be purchased from their Abeyance Realm counterparts, including Charms, Infusions, and Augmentations. You will also need T1 Essence to add Uncommon-tier upgrades to your gear, which increase its stats. Basic items cost 40 T1 Essence each to upgrade.
