In the run up to the release of the new expansion Lightfall on the 28th Bungie are making the witchqueen expansion free to everyone for the next week (23rd to 27th) that includes the campaign which if you complete you can get the weapon pattern for the Ostio Striga exotic SMG a very unique weapon that spreads poison.
The campaign is fairly easy to play through some 8 or so missions so if you have the time to go through it and get the weapon it's a good tool to have in your arsenal.
I think if you play through it on legend (the harder difficulty which is not super hard) you can also get an exotic armor piece but I'm not totally sure on that. You get double rewards for the legend missions anyway.
Maybe we can do it for SNG this week?