So I've been up and about writing my own little novel and I noticed that the word's "mage" possessive form comes up as a mistake. The example would be the following — "mage's tower". The word "mage" itself appears to be in the LibreOffice dictionary (English UK) yet the possessive form seems to be invalid. Though now as I'm writing this question while using Google Chrome with the spellcheck of English (UK), "mage's tower" seems to be valid. So I'm very keen on knowing whether I'm using the word "mage" wrongly or if it's an issue with the software that I'm using.
I believe mage's is the proper possessive form of mage. I looked around the web, including several online dictionaries, and I can't find any source to confirm that, but I'm not finding any alternate possessive form either, which I would expect to be listed if there was one.
Google N-gram viewer does show some use for "mage's", though funnily enough only in recent decades. To calibrate this, I searched for a few other nouns that might appear in similar contexts - "horse", "wizard", "apprentice", "king", etc. - and found that the possessive forms seem to be used, on average, about 1/20th as often as the corresponding root (bear in mind this is a very rough analysis). The relationship between "mage" and "mage's" is similar, with a roughly 23:1 ratio. Going by the hypothesis that possessives are often used about 1/20th as often as their nominative roots in contexts where the word "mage" would appear, this suggests that when authors want a possessive form of "mage", "mage's" is usually the one they use. If there is another possessive form of the word, it is unlikely to be dominant.
In any case, it's quite common that correctly spelled words would be marked incorrect by an automated spell checker. The checker in LibreOffice is based on a list of about 64,000 words and some simple transformations that get applied to those words to produce, say, plurals. This falls far short of capturing the full diversity of English. So when you see a word marked incorrect, don't be too afraid to just right click and "Add to dictionary". It's probably worth checking to make sure you didn't make a typo, and you might even want to quickly double-check a separate dictionary, but if you check it and you really think the word is correct, it's more likely that you're right than that the spell checker is.