From the start of the new story arc 'Death of a King' and leading up to Aquaman #19, we have been seeing a mysteriously ancient "Ice King" with a wrathful grudge on Aquaman. Now after given hints, I'm beginning to think that this once king of the seven seas is Poseidon. Anyone else believes so too?
Here's a scene in the epilogue to 'Throne of Atlantis' where he first makes his appearance.
Here is a scene in the beginning of 'Death of a King' as the forgotten king creates an earthquake in Antarctica which Poseidon is known to do, specifically out of rage.
And here's the latest showing of the 'Ice King' in the ending of Aquaman #18 where he demands Mera to lead him to Xebel so that the Seven Seas could be is again.
That being said, it definitely stirred up the probability of whether or not this is Poseidon, as he is first seen in Wonder Woman #5 with a different appearance. Triton, his son who once claimed the Seven Seas under his authority is also a name to drop as well. He became enraged with Aquaman once he found out that his people thought of him as more of a king to Atlantis than he was. Thus, this could go either way.