The Jomsvikings are the only Norse holy order in-game. They will form by asking to build a castle in one of the provinces in the Duchy of Pomerania or Mecklemburg if the province is ruled by a character of the Norse faith (pagan and reformed) and the year is after 920.
They can be hired with piety by any ruler of the Reformed Norse faith (pagan if not reformed yet), and can be used for free if defending against non-Norse rulers.
They will only fight against infidels and heretics, they will not take part in a battle against a Reformed Norse ruler.
By granting the order a province that you control (As King, Emperor, etc) you can recruit them at less than 10 piety. To do this give them a title then ask them to be your vassal.
The semi-legendary Jomsvikings were said to have fought as mercenaries for Christian rulers as well as Pagans, despite the leader, Palnatoke , being a staunch defender of Paganism and "the old ways."