Thou not much of a lore person, especially for FF14, & the lore-fanatic's have explained anything in-game givin about it my only thoughts are more "Headcannon" then lore: the 13th shard that became a "Void", thus can't be rejoined & has nothing of value to Zodiark, is FF14's "Sea Shore" or "Reality Wall" to the Void that exsist's between all FF games (hence the multi-game lore about several enemy/ally NPC's being the same person). this is the only asnwer I got that can make "FF14's Void" & the "FF-Multiverse Void" have interconections (to make my idea/theory clearer the 13th shard is the "newest" puzzle piece on the inter-games Void's outer edge allowing FF14 to be entered by it)

Yes I'm probably crazy & I didn't answer the question (mostly cause the lore-masters already did) I just felt I'd say this & see if it had any merit