I have heard somewhere that 'die' can take on the role as a pronoun. But I can't remember if it is for 'Sie' of for 'sie' (they); and can it only be used at the beginning of a sentence if what I've heard is correct?
As a demonstrative, not as a personal pronoun. It is the same as English "that". Actually not only "die" (feminine and plural) but also "der" (masculine singular) and "das" (neuter singular) as well as oblique forms.