Leaders
There are 7 main leaders in the game as of now and many more to be added. The leader you pick will determine what strategy you use. Each leader has a unique leader tree and has some special units which only they can use. These powers and units make a huge difference in the fight and turn the tides of the battle. There are even some strategies based around just the leader skill tree. As well as this some leaders do much better with certain strategies than others. For example the Shipmaster's teleportation skills aid scarabs quite well in battles since you can just teleport them out and bring them back in when needed. Also all leaders have a hero unit which you can only make 1 of and is unique to that hero. So I couldn't make Atriox's chosen if I was playing as Decimus.