Well... there isn't any.

Slightly hyperbolic, but basically true. It's not really any great surprise, but almost all of the background (and artwork) in the 8th ed. Codex: Necrons is copy-pasted from elsewhere. There are minor new additions, but nothing of any great significance in the grand scheme of things.

The galactic map shows the territories of the Arrynmarok and Sautekh as separate again, in contrast to the map from the rulebook. Presumably the rulebook map was an error then? - there's no indication that the Arrynmarok have become, or been, clients of the Sautekh anywhere else.

As a result of the background for the Deceiver and Nightbringer being in large part lifted straight from the 3rd ed. Codex, the C'tan turning upon one another at the conclusion of their war against the Old Ones is back in print in a current edition publication - further illustrating that the 3rd edition background wasn't [wholly] invalidated with the 5th edition Codex as many claim(ed).