In Skywind, a lot of our worldbuilding is experienced directly, rather than narrated, and there are also a considerable number of books and notes written from an in-universe perspective. However, some additional context is occasionally given by an unseen and unheard narrator. Our new item descriptions are one element of this, but a narrative voice also existed in original Morrowind writing and was used for asides in books that were not expanded on or illustrated for various reasons. For example, many gamers might remember the brief and amusing description of explicit content given in square brackets inside Boethiah's Pillow Book: "[No words can describe what you see. Or what you think you see.]"
This imaginary narrator is singular and consistent and though they cannot always be trusted to be perfectly objective and nonpartisan, they should be imagined to be as neutral and consistent as any narrator or D&D dungeon master ought to be. They are not overly informal or casual, and do not use colloquialisms, slang, l33t speak, non-standard orthography, or terms or phrases unique to real-world cultures or regions, or refer to real-world pop culture.
When writing text that explores the world experienced by the player, but which is not mediated through books or dialogue, you should consider carefully whether it fits the narrator's neutral voice, or whether what you are writing indicates a distinct character and perspective. The latter is only acceptable in instances when a framing device can be created, such as a clearly-marked quotation (real or invented) added to an item description, which can bring that non-neutral voice in-universe.