Poor Diana

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Why can't the torii be floating behind him? Bloodmoon theme is kinda magical, so there is nothing that would forbid that.

Maybe it is jumping to conclusion, but you have to admit there is some evidence which might point in Yoricks direction.

1. The torii (even if you doubt it) is still imaginable as the thing on his back, floating behind him. I think it has some similarities in design, especially the half circle pointing upwards. Have to admit it is a stretch, though.
2. The torii is covered with black mist. That is a big part of Yoricks base model design, having that mist cape.
3. The text "blood in the heavens, blood on the ground" kinda pointes in the direction of 2 skins and Yorick as a grave digger would fit the part "blood on the ground" pretty well.
4. Bloodmoon skins are highly associated with 2 regions: Ionia and the Shadow Isles. Jhin is from Ionia, Yorick would fit in that too since he is from the Shadow Isles.

Nothing proven but there are some arguments that Yorick could be a candidate for a bloodmoon skin at Jhin's side.

Let's discuss some things. Yorick's gravestone on his back does not float on his base model. To do that for a skin, he would have to be given an 1820 skin that modified his model at the very least, and so far, the emphasis is clearly on Jhin. They wouldn't vaguely hint at Yorick if he were the big skin for this set.

The Torii is covered in mist yes. But Yorick isn't the only Shadow Isle champion, and the entire island is covered in the mist. It is more thematic for the island, and not exclusively tied to Yorick. Since, as you've said, the Blood Moon skins are Ionians and Shadow Isle denizens, this doesn't particularly point to Yorick.

The line, Blood in the Heavens, Blood on the Ground is not tied to Yorick either. Jhin is not celestial, nor is he associated with the heavens. If the Blood on the Ground was pointing to Yorick, then that would mean Blood in the Heavens would point to Jhin. This makes no sense at all. If anything, Blood on the Ground points to Jhin and his homicidal artistic tendencies. This is a weak argument for Yorick.

Blood Moon has been associated with both regions, yes. But there's also Zilean. And there's also many other Shadow Isle champions who don't have Blood Moon skins. Hecarim springs to mind, and he's proven to be a helluva more popular champion than Yorick, and would therefore sell as a skin a lot better. There's also Maokai, Syndra, and Zed who don't have Blood Moon skins. Zed and Syndra are also highly popular Ionians, with Syndra being lacking in skins as well. Furthermore, locking two entire regions of champions into a skin line is just obligatory and stale.