The Plica Semilunaris (or “third eyelid”):
Per millimetre, this little triangle of mesh represents the most time I’ve spent on anything to date. It had to be thin enough to be wedged in-between the eyeball and eyelids, and appear to be touching them without actually touching them (otherwise there’s a conspicuous black artefact at the point of contact), while being thick enough to withstand all the forces tugging it around without intersecting itself or the tear line, which is sliding over it and contorting with the eyeball and eyelids.
This was initially facilitated by extending the plica geometry so that it travels more evenly over the eyeball, the upshot of which is that it alleviated the darkness in the earlier version, which it seems was caused by the Random Walk SSS exceeding the geometry thickness. I bolstered this by adding a greyscale map to reduce the SSS depth in the thinnest areas, and softened the sharp leading edge by adding a transparency gradient and mixing in a Glossy BSDF to retain highlights on the now invisible edge.
While I was at it I reshaped the tear line to cover the caruncle, to stop it from tangling up as it follows the plica. Since I had extended the plica, the bump on the tear line (which is tracking the eyeball) needed to slow down where it’s overlapping the plica, so it looks like the meniscus is moving with it instead of streaming over it. I faked that by dissolving it into a stationary bump at the caruncle.
Other improvements include refinements to the skin material, reshaping of the eyelids, a makeshift bump on the eyelashes (more about that here), a fix to a perplexing eyelid driver glitch, and more UV revisions than I care to remember. And I finally got to make use of that Iris sculpt I made back in 2013!
Incidentally I had to re-render the upper eyelid in 2.92 because of too much splotchy flickering in the SSS, which neither disabling denoising nor increasing the samples would fix. It’s still visible, but not as pronounced as in later versions, which is a tad concerning…