Some of Elden Ring Nightreign's Remembrances are playable snippets of a moment from the character’s past. When you launch into a playable Remembrance, there will be a hazy dreamlike border around the screen, and you’ll be transported to the location of the memory.

-- Source: https://www.ign.com/articles/elden-ring-nightreign-is-taking-a-unique-approach-to-storytelling-ign-first