Well... I'm the kind of person who will start reading a fic on AO3, say "that can't be right," and go down an hour-long Wikipedia hole about historical lubes.
Most natural plant-based oils may be used as lube, though they're messier and not as good as modern water-based or silicone-based lube. Just remember that oils aren't good with latex ā but your knights probably aren't using latex condoms, anyway.
Historically, a common sword oil was linseed oil, which is natural, plant-based, and food-safe. So, again, a fine choice.
Other historical options include other plant- or vegetable-based oils, which are probably fine. Some also used animal oils though, which is not fine and can lead to infections. Avoid that shit š
For traditional Japanese swords, choji oil was used, which is made from cloves. High concentrations of clove oil can be toxic, so you HAVE to dilute that shit ā 1% concentration or less. Once diluted though, clove oil is considered safe and can be used to treat and soothe anal injuries. It also has a numbing effect that could help with rough anal sex, but your ass would hurt like a bitch after the effect wears off.
A lot of modern sword oil is just mineral oil, which, while not plant-based, is food-safe and is actually recommended for use in enemas because it's safe and long-lasting. So while it's not great for vaginal use because it can irritate the skin there, it can be pretty well-suited for anal use.
There are a few modern sword oils made from machine oil and motor oil. Those should not go in any orifice whatsoever š
Always check the label before using improvised lube.
Anyway, I hope that helps! I hope your knights enjoy their fantasy love-making ā¤ļø