Question about Forgotten Sword

do you have to have nightfall in order you use this weapon?

No.

If you buy it (or get given it) from someone else you can use it just fine, assuming it isn't customized.

no, any character can equip any weapon in the game as long as it is not customized for another character

ok thanks, thats what i was thinking

ok thanks, thats what i was thinking

btw, thats the same with every other weapon/offhand. only exception is nightfall pwns in terms of inscribable and insigniable items.

i doubt it but, is there a way "uncustomize" ?

Nope, once customized, it's only useable to that person or their heroes