after he dies, yun ends up back in the spirit world. it seems to him that his time amongst mortals was all to brief and passionate, but there's nothing he can do about it- he knows this is permanent. the yun that once was is gone, and there's no way he's ever going to go back. in his quest for power he took up a mantle he wasn't planning to, absorbed powers he really shouldn't have, and now he's stuck here, forever. a punishment fitting of his actions
time passes, and nothing changes. yun isn't used to it- being a spirit, and he has one to help him get used to it. there's no one there to watch over him, it's just him. he gets used to it, it's not like he hasn't been alone before. it's a new type of alone, though, and he's not sure what to think of it
centuries after kyoshi takes her final breath, the moon disappears. Yun can feel it, even though he's never been connected to water and still isn't (after all, he still is a spirit of the earth, the title of world-borer still means something), he can feel it- the devastation of something so integral as the moon. it only lasts a moment, though, before kyoshi's spirit- yun's pretty sure she's now reincarnated into an airbender boy- fixes everything. the moon returns to the sky, and yun, while he doesn't understand it, accepts the new reality as truth.
yue is new to the spirit world- it all happened so fast, one moment she's in Sokka's arms, and the next she isn't. it takes a moment to adjust, and grieve. yue realizes that she is now alone- except she isn't. there are rumors of a boy who came from the mortal world to the spirit one almost five hundred years ago. it's a longshot, but it's the only chance yue has at talking to someone about how she feels
yun resides in a swampland- an area dismal and desolate compared to what of the spirit world yue has seen, but she persits anyways.
yue finds yun in the trunk of a mangrove tree, the green of his eyes casting sickly light across his features. yue is afraid, but she tells him who she is and asks to talk
yun is furious- how dare she come here, a reminder of what he's lost and demand he speak to her. it's been so long since he's been human- why would he care to talk to her about it! it's only a reminder of what's happened to him and so much of what he's lost. her, the close friend of his enemies' reincarnation, demands he speak with her. It's mocking.
yue doesn't understand, out of sheer lack of knowledge. she didn't come to make him upset, she just wanted to talk. in her heart- or lack of one, now- she feels that if they talked they could actually get to know one another and benefit from it. yun doesn't care. he's spent the last 500 years alone and he's planning to do the same for the rest of his (still) new immortality.
yue leaves. she doubted it would've worked, anyways.