Just now getting to the second installment in the War of the Rot (Volume 3 is already out, so I've fallen behind). It's still moving very slowly...at the midway point Buddy Baker and family are still on the run, and they still haven't met the Swamp Thing. There has been one interesting development: Buddy has apparently died and gone to the Red, and the Rot have taken over his abandoned body. There's a nice callback to the original series as Buddy recalls meeting his creator ("He was this skinny, intense, Scottish guy who claimed I was just a character that he wrote in a comic book"), a reference to the famous conclusion of Grant Morrison's run.
Annual #1 occupies the middle of the collection. This works because of the loose ties to the monthly continuity: it features the talking cat (former avatar of the Red) telling Maxine (current avatar of the Red) the story of the last time Red and Green united to battle the Rot. So we get to meet an earlier Swamp Thing, and get a preview of the coming battle.
Issue 9 concludes with the appearance of John Constantine. So the gang's almost all here.
Not just Constantine: Zatanna and Madam Xanadu have also come to see the Bakers. Not to help them, just to warn them. Which gives DC an opportunity to further the worldbuilding around its stable of magical characters, but doesn't actually advance the plot much. Constantine does throw out an interesting clue, though. After telling the family "It's the big green guy you need to find," he follows that with "Wait till she realizes I wasn't talking about Swampy."
Meanwhile Buddy Baker has his audience with the Totems of the Red. They tell him they can't put him back in his body--it has already been overtaken by an agent of the Rot--so they decide to build him a new, stronger body. When he returns his powers have indeed been amped up considerably: he uses them to rescue Cliff from himself, in the form of his former body. The arc ends with Cliff warning them about the impending arrival of Arcane.
The Zero issue is the final story in the collection. It tells the story of the Animal Man's origin, retconning it as an intervention by the Red. The Red knew that Maxine would be the next avatar, so they gave Buddy limited animal powers to prepare the way. And the alien abduction scenario in the original origin is a hoax they cooked up so Buddy could understand what happened to him in more familiar terms.
So the image of the Swamp Thing on the cover is a big tease: we only see him in flashback. But one thing is clear: life requires the Red, the Green, and the Rot, but they must be in balance. The Rot is beginning to dominate the other two, threatening the balance, and therefore life itself. By the end of this collection I felt like that had been mentioned about every other page, but I'm sure I'm exaggerating.