i think it was mentioned or shown that the necklace was that of her brother or her boyfriend(dont know which one), whom both died. it serves as a memory.
tsunade is the firsts granddaughter, the title probably mentiones that they are going to show some flashbacks of tsunade or something that connects with the necklace and all
i dont think that the death god that sandaime summoned has anything to do with it

maybe the necklace allows u to use the sealign jutsu, and not die or something. if thats the case, then we could have tsunade fight oro, and seal his soul completely. that way she would finish what the 3rd started. it makes sense plot wise.
if im correct, yondaime developed the "death god, sealing jutsu," so the necklace might not have any significance with it, but i dont know - its just my speculation
well just have to wait and see in the next ep what happens