Ever since Sigismund was elevated to the rank of first High Marshal of the Black Templars, there are others who have risen to take his place as Emperor's Champion. The practice of having an Emperor's Champion has spread to other chapters, but it is enshrined most strongly within the dogma of the Black Templars.

Each Crusade nominally has an Emperor's Champion, but in practice there is actually one for every fighting force. On the eve of every battle, on amongst the Space Marine host will receive a vision from the Emperor and present himself to the Chaplains. They will anoint him as The Emperor's Champion.

The Emperor's Champion may be included in your Space Marine army as an HQ choice.