BALANCE:
The class is clearly meant to be an evil counterpart of the Paladin, therefore it is easy to compare it with the core class for balance purposes. With the Paladin class it shares HD, BAB, proficiencies, saving throws, skill points, a very similar class skill list, and several special abilities with "switched" alignment or effect (Detect Good, Smite Good, Aura of Despair and Rebuke Undead; plus Dark Blessing, but 1 level later). A Blackguard casts spells from 1st level, but being a PrCl it basically starts later than a single class paladin.
On the overall it looks very close in power with the Paladin
FEATURES:
If you consider than most of the abilities are evil replica of the Paldin's ones, there is nothing much really new or unique to this prestige class at all, which is a pity. On the other hand, the whole set of features makes very sense for the kind of character it is supposed to be.
Fiendish servant is somehow more versatile than Divine Mount, since it can be also a non-mount. The spells list is very well designed, and in this case I agree that the list doesn't really need to be longer, since the Blackguard is a divine spellcasters and knows every spell on his list, but the character is definitely more combat-oriented.
It is very nice the idea of giving bonus features to an ex-Paladin, who would lose all his speical abilties, including mount and spells. Becoming a Blackguard, the "fallen" gets extra abilities that compensate the loss of the previous. Given tha fact that he keeps hit points, BAB, ST (except Divine Grace), skills and feats, and also considering that the Paladin is quite front-loaded, I think that generally the higher level the Paladin was, the better the retribution (and therefore, the temptation) to become a Blackguard. Probably it is still better to just stay Paladin, but if it happens to fall...
FLAVOR/RP:
Definitely one of the easiest NPC class to use for a villain, or for the villain's personal bodyguard. The idea of him being a fallen former champion of good may be the centre of a whole interesting campaign.
As every evil character is more difficult for the player to roleplay, and therefore may have less appeal as a PC.
REQUIREMENTS:
IMHO, the requirements for this class are not well designed. Specifically, I can't see the point at all why Hide skill and Sunder feat should be required at all, and even Cleave leaves me some doubts, although it is more acceptable. If they wanted some general fighting experience to be needed, BAB and a minimum number (e.g. 3) of feats from the Ftr bonus feas list - not necessarily taken AS Ftr bonus feats - would have been enough, along with maybe a rank in Intimidate or Bluff or even Diplomacy (but why Hide?).
DESIGN:
Generally, it is quite well designed; it would have been IMHO nicer to get stronger compensation from former paladin levels. After all changing the side in the whole world conflict may have already its own big drawbacks, and it should be compensated.
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OVERALL: 8
