Currently, the wiki has individual articles for even the most minor gag NPCs, unused items, and an unused spell. Therefore, I propose creating an another one, for the unnamed Ralsei's fire spell. Although it is used in only one cutscene and is not yet available as an option in the battle menu, I think it might deserve an article:
The cutscene is implemented as casting a spell (firespell) in the game's code.
There is some foreshadowing of Ralsei being capable casting fire magic (depictions with fire in the prophecy and the LightUp ACT), which may be interesting to the reader, but would feel out of place in the main Ralsei article (unless placed in a some brand-new "Powers and abilities" section).
The places that mention this spell and search requests would benefit from an entry point to redirect and link to. I have already added a non-standard mention to Ralsei's Spells section and an entry about the firespell to the Spell article, but the first is too brief, and the second is too technical.
There is a decent probability that this spell will receive more significance, or even become an usable spell, in a later Chapter. I do not think there is a reason to wait on this, when there is enough information for an article longer than Chairery.
You can read my draft here. I am unsure whether the unused sprite mention in Creation and development is noteworthy, but if it is, it is an another reason why this should be an article.
I think giving it an article separate from Ralsei is not wise until it becomes usable in battle. I would simply state that Ralsei has fire magic in his own article. Thinking about it, a few wikis have a "Powers and abilities" section on their character pages... this would be a great to place this information.
For the moment, I think I would reclassify the article as a plot element rather than an actual spell. The spell is currently not selectable in the battle menu, and there is currently no indication that it would actually become a real spell until more Deltarune content releases.
I supposed that this article can be a combination of a plot element and unused content article, but keeping the technical firespell mention in Spells and reducing this article to be about plot can be an alternative for this proposal.
Although that Rude Buster's description with "Rude-elemental damage" and unused dialogue about Noelle controlling the "Ice element" can warrant such a section for Susie, Noelle, and Ralsei, I think that this will quickly turn into conjecture for other characters. Noelle's section would be have to be based on unused dialogue, which is questionable too.
Apart from Susie and Noelle, this will require the wiki to make up its own classification. For Ralsei, for example, this will either require to conjecturally connect his magic to the Fire element or use the vague term "fire magic". For other characters, it will lead to uncertainty about what even an "ability" is, and whether it is really unique to the said character. The Element article already mentions the "Rude" element and the "Ice" element from the unused dialogue mentioned earlier here (without specifying that it is unused), so at best, the new section should be about elements only.
Ralsei's stunt in Chapter 5 is explicitly classified as usage of a spell, both by the code and the sprite's name, so reducing it to be just usage of an "ability" or elemental magic will override the game in favor of the wiki's arbitrary classification (a spell has to be usable in battle).
On the other hand, the game explicitly calls it a spell in the code, and the decision to reduce spells to only something that can be used in battle is a bit arbitrary. Although until now all spells in Deltarune coincided with the ones in gameplay, this is an exception, and I don't see how Ralsei's decision to hold back makes a spell not a spell.
A spell is a gameplay mechanic, though? If it cannot be used in battle, it's not a Spell, as in, the thing Rude Buster and Iceshock are. Determining an article's scope by how many things are describable by its name is not a good idea. Else, we're gonna have to include Bibliox's "spells" too.
Deltarune is a story game that utilizes spells not only as a gameplay mechanic, but as a plot element too. The descriptions of their frequent use in cutscenes in their articles already making the supposed gameplay mechanic articles not as gameplay-only.
Until Chapter 5, all spells used in cutscenes coincidentally were also available in battle, making the gameplay mechanic definition plausible. Chapter 5 introduced at least two plot elements that behave and look identically to a spell being used in a cutscene. The second "spell" I am talking about is Blue's "Healing Rain", which is utilized several times in cutscenes, and has interestingly similar flavor text to the party's spells when casted in Yellow's battle.
To me, their existence suggests that the definition of the concept of a spell in Deltarune is supposed to go beyond being an option in the battle UI. Even without these, Deltarune features spells that are purposed to improve the story more than gameplay, like Susie's healing and SnowGrave. Bibliox "spells" are only mentioned in Bibliox's descriptions, and we don't know what they are and how they behave, so there isn't much to include about them.