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An egg from Tsubasa? I forgot when I stopped Tsubasa. Somewhere around when the story started getting too confusing and had like 80 Sakuras running around.

Tokyo arc, the one that had an OVA titled "Tsubasa Reservoir Chronicles: Tokyo Revelations". Actually, this is before we know there's 80 Sakuras running around. A few refreshers in terms of what happened and what was in this arc for you to see if you recall this arc: twin vampires that Seishiro was hunting for who were crossover characters from another CLAMP series, it rained acid rain all the time and the only place in the city unaffected was the government building where underneath was the only remaining pure water left due protection coming from one of Sakura's feathers, real Syaoran came into the story in this arc and then the clone went crazy, and then Sakura went action girl during this arc to look for this item Yuuko to be able to save the world dying from acid rain. This item was an egg. I think it looked like a shiny rock in Tsubasa, but in xxxHOLiC, somewhere maybe around after the Jorougumo chapters- they address it as an egg, and when it arrives, it was one existence in another world and in this world, it became two eggs: one that hatches into a bird that brings good luck that Yuuko gives to cancel out Himawari's bad luck, and the other is unknown right now; but it's going to serve as a major plot device in the story. They also compare it to how Syaoran and Watanuki are the same existence, but Syaoran turned back time to avoid Sakura's death from a seal done by Fei Wong Reed, where instead of real Syaoran by Cardcaptor Sakura's Syaoran and Sakura, Watanuki is born. Confusing stuff, right?

[spoiler=If you don't plan on reading the confusing chapters and intend to skip to chapter 188]
The egg's apparently given to Doumeki to use on Watanuki, but nobody knows what it does; and Doumeki constantly thinks about if he should use it or not within the "Rou" half of the series (chapter 188+) and the chapters before that where Watanuki decides to become the new store owner when Yuuko dies.
[/spoiler]