Starting with Saryn Prime, every new prime warframe has come with a cinematic trailer, narrated by Executor Ballas. His narration reveals tidbits of information about the nature of the warframes, and so they are contained here.
Saryn Prime, Vauban Prime, and Nekros Prime all had their prime trailers released the day their Prime Access went live. When Valkyr Prime was released, the animation team was busy working on The War Within and were unable to devote resources to a prime trailer, although the marketing team did put together a short promotional video. The Banshee Prime trailer was released with Banshee Prime, and the Valkyr Prime trailer was released almost a month after that. The Oberon Prime trailer released with Oberon Prime Access, but the trailer for Hydroid Prime was not released, likely due to animation resources being used on the Plains of Eidolon and related content. The Mirage Prime trailer was released with Mirage Prime. Since then, no trailers have been released for subsequent prime warframes. Finally, the missing Hydroid Prime trailer was released at TennoCon 2020, roughly three years after it would have normally come out.
The first Prime Access of 2021, Octavia Prime, was released alongside a prime trailer, the first since Mirage Prime in 2017. Nezha Prime, which was released before Octavia Prime but after the Hydroid Prime trailer, does not currently have a prime trailer. The prime trailers resumed, with the following Gara Prime also releasing along with her prime trailer.
Discussed in: Prime Warframe Cinematic Trailers