Another theory is that it was in Hell, Shadowheart or someone brought it back from Avernus during the Descent, which is why the nautiloid was raiding Elturel, picking up Shadowheart who had it in her possession, the question remains how Lae'zel got aboard though.
This seems like solid theory ...
Especialy if Karlach will be Origin character, in order to give her the same start, she have to be in the Pod during attack on Yartar ...
That would mean those Gith who were attacking Nautiloid were not attacking it "randomly" but were chasing it from its previous visit in Avernus ... where they draged Shadow, Karlach and possibly even Lae'zel.
Even tho Lae'zel was probably kidnapped elsewhere, since she would probably mention some tie to either the box, or Shadow allready ... and she seemed quite unaware of both.
