Welcome! The Varangur are a lot of fun top play. I’ve played them a bit (links to battle reports can be found here and the blog has hobby posts as well. The forum also has some more discussion on the battles here.
A little more on the army’s fluff and background can be found in the game’s supplemental books, namely the Uncharted Empires books from both 2nd and 3rd Edition, but @Dubren definitely hits the highlights.
If it helps, here are some old pics of mine covering those units.
My Fallen, hobbied up ages ago. Some Skorne models from Warmachine. Being able to run cool-looking units from other games is a nice plus for Kings of War.
My Reavers, for now. Lots of more barbarous options around though these days.
Some Magus Conclaves. They are indeed small groups of casters, but working together are able to toss around some devastating magics, functionally similar to a war engine. Having Nimble and Steady Aim, these duders can move around and still shoot, unlike most other war machines, making them a lot of fun to field.