I personally don't think so. We should have all the shadowcrystals we need to make the Shadow Mantle by Chapter 6. Assume we can't return to Castle Town before we fight the final, it'd be pointless aside some extra challenge before the end, which, while poissible, I don't think toby will give to us.
Erm actually the shadow mantle is needed for the boss in chapter 3
Anyway yes cause seven shadow crystals puts them up there with the other mystical artifacts that come in seven
Could be that the Shadow Mantle is strange. We have all these teasers about it, and based on the Chapter5 subpage of Spamton Sweepstakes, and Chapter 3, the Shadow Mantle might be the divergence point, just as big as the Ch 2 Weird Route,
Someone else stole it and you’re not supposed to be able to get it.
Maybe you actually have to get it from Chapter 4’s secret boss and then go back to Chapter 3? Secret boss items and shadow crystals persist across saves.
This aged like fine wine.
It's actually just a random route in Chapter 3 :/