I posted this on the subreddit earlier and was recommended to post here about it.
This solo event just appeared on my alt account (which has only beaten act 2) but isn't showing up on my main account (which is Cav). I faintly remember this being announced but I could 100% be wrong with that. I've checked here and the subreddit and can't find it mentioned anywhere. Just wanted to see if anyone else had it.