This is another event that I've repeatedly managed to miss - until last time, when I was at least aware that it was happening but decided to ignore it at the time, just to kick myself later for missing out on a cool mount.
I thought that it was set to recur a week or two before the Protector's Jubilee (Neverwinter's birthday), as the original Neverwinter intro cinematic also starts with the city under siege. However, for some reason the event is live again now, so clearly I was wrong about that.
Either way I've been appreciative of another opportunity to try and get that armoured horse. The event neither asks nor offers much, consisting of a small combat area with a couple of cycling heroic encounters. Since it's all scaled to level 60, the mobs pretty much explode in the presence of level 70 characters - I often can't even figure out where an event end boss has spawned before he's already fallen over dead. You can do as much or as little as you like, as the daily only requires you to complete two encounters. It's easy XP for levellers and a great farming ground for stronghold vouchers though.