Leaders are most important person in Halo Wars 2, leaders plays an important role in your overall strategy. While all leaders can be used offensively and defensively, some leaders offer special Powers or unique units that complement specific tactics.
Halo Wars 2
Blitz is a competitive game mode that offers a variety of public and private match types supporting everything from 1v1 tests of strategy to chaotic 3v3 battles. In Blitz every leader offers different starting armies and exclusive cards, different leaders lend themselves to different strategies.
Phoenix logs make for some good reading. Phoenix logs is another collectible in Halo Wars 2, which you can find in the different missions, collecting all of them will gives you some extra information about the game story.
Skulls can be earned by completing different task in the specific mission; you can have them as optional objective. Once you compete the objective you will earn the skull. Earning different skulls will give you some extra bonus effects which can be very useful throughout your mission progress.