Stellaris is a new breed of 4X space strategy by Paradox Interactive, labelled as a new breath of fresh air in an otherwise heavily cloned genre. The game sets itself closer to the likes of Master of Orion than some of the latest titles like Civilisation or Endless Space, and crams in masses of content supported by some very simplistic basic mechanics. Stellaris aims to make those week long skirmishes easy to approach, tossing aside overcomplicated resource management, instead focusing heavily on sheer species development and your inevitable ownership of the galaxy. The whole game is played purely through a single customisable skirmish mode, so there's no real campaign mode per se.
The storyline in this game is pretty much however you decide to make it. Your choice of race will probably directly affect your personal story, as some races are designed to be more diplomatic, while other are outfitted with passives extremely orientated towards war. The game is filled with numerous sub-species, like alpha aliens and pre-space age planets, so you'll come across all this life in the galaxy and be tasked to understand and learn about them, leading to eventual technological advances. You can craft enormous alliances, wage wars that are won not by simple domination but by numerous successful campaigns against conglomerates, or even stealthily infiltrate planets and overthrow their political leaders. There's always the option to be a highly valued passive member of an alliance however, funding another race in their quest for war while you reap the profits. There really are innumerable possibilities of how you can win or lose a skirmish.