Im like 80% sure this will be Mega Darkrai's ability
They made it slower but also more bulky. For me that only means one thing. They want to buff its signature move Dark Void. And instead of buffing the actual move they will buff it through its ability, so the likes of smeargle wouldn't be able to abuse it like we have seen it before.
The ability would be a combination of bad dreams + for the ability for Dark Void to always hit. Now if Darkrai got the Z mega treatment and received a 150 ish speed stat this would obviously be broken af. So they intentionally made it slower and more bulky to to account for that. Lmk what u guys think