Primnox generates it for you, once, inside the app — at the moment you create your vault. It is 24 words, and it is the only thing besides your passphrase that can open your data. Write it down offline. If you lose both, the data is unreadable and nobody, including us, can recover it.
This page used to generate one. It shouldn't have, and it no longer does. A phrase made here would not have been the phrase your vault was actually encrypted with — it was never sent anywhere and never could have unlocked anything. If you wrote one down from this page, it is not a backup of anything. The real one is shown once, in the app, on the screen right after you set your passphrase.