I heard on the forums that a few people had lost umbra in the SI pack, so if you have lost it or you dont want to loose it read here.
It happened to me the other day but i found out a way to get it back !
I lost mine when i was sent to jail once you fight your way through the dungeons and you open the chest to get your items back umbra isn't included in the items, but technically you dont actually loose it, it just becomes invisible.

What you want to do to prevent loosing umbra is set it to your shortcut pad (mine was on up if it makes any difference), now when you get locked up all your items are gone but if you check your shortcut pad it will still be in there! all you need to do is use it for a while until it gets nice and damaged, then use a repair hammer and pow! its there in your repair list once you have knocked it up to 100 or 125, then it it will magically appear back in your inventory!

I know you get some more powerfull weapons in SI but they all depend on enchantments, umbra is the only weapon with a high power even when it has no charge! still the best one-handed sword imo.