“Look out, everybody, things are about to get awesome!”
Nyriad is a non-player character in Borderlands 3. The Vault Hunters never meet Nyriad in person, but find her writings scattered across the Borderlands.
In an Eridian glyph found in the Tazendeer Ruins, she describes her self as "the last to learn the Eridian language from the Eridians", hence explaining why her voice permeates the glyphs when they are translated; this is likely related to the Siren ability to imprint objects with psychic records of traumatic moments or milestones of their lives (See Childhood's End), and judging from the large number of them scattered throughout Borderlands 3, Nyriad likely gained a mastery of that ability to do so at will.
An Eridian glyph recording in The Pyre of Stars that Nyriad leaves behind has her declare that "it was here that [she] killed them. A billion lives to power a machine," which cements her identity as the Siren who powered the Pyre when the Destroyer was sealed away the first time. From her words, she is haunted by her actions, even if they had saved the known universe, at least temporarily, from the Destroyer's wrath.
Nyriad also explains that after sealing The Destroyer, she traveled to Nekrotafeyo and sealed herself in the Vault, fearing that her power was too dangerous to be used. It's implied that when Typhon DeLeon and his wife Leda opened the Vault, Nyriad's power escaped and ultimately manifested in their children, Tyreen and Troy Calypso