In Episode Ignis, when Ignis has the Ring of Lucii, after Ardyn warns him of the danger of putting on the ring, Ignis notes that a Glaive was able to use the ring.
From my understanding, the Ring of Lucii has always been worn by royalty with one exception being Nyx Ulric, one of the Kingsglaive (or Glaives for short) who in Kingsglaive had worn the ring and been able to use its power to fight General Glauca and awaken the Old Wall.
Yet Nyx using the Ring of Lucii happened during Insomnia's fall when Noctus's crew were out of the city heading to Altissia and had no knowledge of the events in it beyond that the Empire attacked and Noctus's father had been killed.
So how did Ignis know about Nyx having worn the Ring of Lucii?