It's confirmed fake, it was made by the infamous Red Shirt Guy, but he said on twitter that it doubles as his actual predictions.
I think the slime saber doesn't have to do with them. We have two encrypted mounts with serpent noises that are likely their racial mounts, though I don't know what they would be. He just chose the slime saber because it is also encrypted on the PTR... though I think that's because it is supposed to be a surprise, not because it is tied to the Tirisfal/Darkfallen stuff.