Autarch
An Autarch is an Eldar of the Craftworlds who walks the Path of Command, a specialized path which studies all the different tactics a warhost of the Eldar Craftworld uses and then brings their different skills and abilities together into a single force that maximizes all their strengths and covers their individual weaknesses.
The path of command is the path that studies tactics, make plans and guides the Eldar in their war efforts. Those who walk it are referred to as Autarchs. Before becoming Autarchs, these Eldar would have studied in many of the different war temples of Khaine, mastering each path before choosing another. What this means is that Autarchs will have a range of varied skillsets and will be powerful warriors as great leaders.
On the board this allows you to select all manner of wargear to kit out your Autarch and create your own unique leader for your host. No two Autarchs may need look alike and the modeling possibilities are endless.
They also make terrible decisions.
Prince Yriel of Craftworld Iyanden is perhaps the most famous of the Eldar Autarchs and is a swashbuckling Eldar pirate....in sphess! He beat up a bunch of nids though so he has some more balls than most Eldar. Interestingly, Yriel has never walked the Path of Command, but achieved his post of Autarch due to walking the Path of the Mariner.