Warhammer 40K: Imperium is an 18 issue publication in magazine format.
Further back there was a similar publication called Warhammer 40K: Conquest, and this one was mostly providing Primaris and Death Guard figures, although there was other associated items provided as well. There were various articles like fluff, modeling, painting, etc. However, this was only available in the U.K., and not elsewhere.
With Imperium, the format is the same, and it has started to be produced in the U.K., although it is expected that it will be available in the U.S. too. The website that folks can see the offers has both a format for the U.S., and the U.K.
Just a quick heads up on Imperium ... this is mostly centered on the Indomitus box product, so Primaris and Necrons, although they will also be making available Sisters of Battle from their set, and some Ad Mech figures, too. Terrain sprues are also going to be made available with some of the issues. There will again be fluff, modeling, and painting articles in the magazine portion of the subscription.
Cost is $4.95 for the first issue, $11.95 per issue beyond the first, 1 issue per week for 18 weeks, total cost is $208.10.
It really looks like quite the deal, especially if a person hasn't bought the Indomitus starter set, and are interested in Primaris, and Necrons with Sisters and Ad Mech thrown in along the way, plus terrain sprues ... and things like paint and modeling tools.
Here's a link to a video with a bit of explanation ...
https://www.youtube.com/watch?v=ZWaFw7RxD2U 
