[Alneyan]

Holy/unholy units (or commanders) are your sacred units, particularly devoted to your cause... you get the picture. In game terms, they cost half upkeep (1/30th of their gold cost), and can be blessed. Blessing them gives a +3 morale bonus, and other effects depending on the magic you picked when *creating* your Pretender. Level 4 in any magic path gives a minor bonus, level 6/8 increases that bonus, level 9 gives a big bonus, and level 10 increases the level 4/6/8 bonus. For example, Fire 10 results in a +5 bonus to attack, plus flaming weapons; Fire 4 would only give +2 to attack. Stacking blessings can work out quite well, and this thread is dedicated to coming up with ways of boosting the power of sacred units via blessings. There is no difference between Holy and Unholy units here, except the colour of their candelabra icon.

Holy/unholy mages have access to Holy or Unholy magic; they are your priests, in other words. All priests can call back your Pretender (you need 40 priest levels to call the Pretender back), can build Temples, and require a Temple to be recruited. Holy priests can preach to boost Dominion in a province; Unholy priests can reanimate creatures. Holy and Unholy priests can also cast some spells, depending on their priest level: Banishment, Blessing spells and Morale spells are the most common for Holy priests, and Unholy priests have different spells depending on their nation. All priests are also sacred, including the Unholy priests of Broken Empire, who lack the Sacred icon, but still cost half the upkeep.

So higher levels in priest magic are only needed to get access to the better spells, and for preaching/reanimation purposes. On a related note, Prophets are effectively priests, but do not cost any upkeep, and will be either Holy or Unholy depending on your nation and on the Prophet (Unholy priests stay Unholy when they are priests, and vice versa).