Thanatos doesn't like how bright the surface is. Yvette doesn't really get it, having fond memories of bright summer days and a love of gardening, but she's noticed his discomfort. There is an obvious fix for this problem, especially when one is a goddess with the ability to materialize objects from assorted matter. A simple modern invention associated with designated cool guys and beach days: sunglasses. Less obvious is how to make the planned pair of sunglasses work with his aesthetic. She can hack together something-like-transitional-lenses with judicious application of 'I am an eldritch god of bullshit,' and has enough design sense to tastefully match his theme easily enough, but, well. Glasses aren't really a fashion statement in Ancient Greece. She really doesn't want to be the one who made Death himself look a bit like a dork to the other gods. If he wants to rock the pair of glasses, good for him (he'd probably be hot in them), but she'd like it to be his choice, not something required for him to be comfortable doing his job.
Hades turns out to have a helmet of invisibility. This does not blind the user, and somehow makes the entire wearer invisible even though it's just a helmet. This is probably through the Greek version of her eldritch god of bullshit clause. Sure, okay. If there's something like that here, she can probably make sunglasses that turn invisible, but still provide their benefits to the wearer. If she gets to look at the original instance. This requires a special requisition form to Hades that she has to half write herself, which of course needs to then be approved before she can even use it, and then a permission form to please let her copy a bit of it for use in making an item for Thanatos. The Lord of the Underworld thinks asking permission before she makes things is a bit much, actually, but she knows what copyright incentives are, and points out that if she ran off and copied everything she ever got to see, no one would ever show her anything. He grudgingly acknowledges the point, and signs the damn form. She gets a small degree of petty satisfaction in out-bureaucratizing lord bureaucracy himself.
Then she gets to actually doing it, which is... hard. It's hard. She just actually can't make something as powerful as Hades's Helmet with the materials she has available to her. Most of what she has (and is) is a bit too grounded in physics for that sort of nonsense. No turning anyone invisible with a tiny helmet for her. But, with a bit of clever manipulation of light, a helping hand from a slightly amused and indulgent Nyx, and judicious application of some of her less physics based power from eating the fire river, she manages it.
"Oh Thaaaaaaanatoooooos~" she singsongs, upon his return to the House of Hades. The piece of her that had been accompanying him on his death dealing reintegrates with the rest of her, and her full proper form rises out of starscape in front of him with a massive smile.