The Undead set of powers isn't perfectly balanced, but let me represent a few points here. (I'm taking everything below under the presumption that these are PCs), this is pretty long, so you can skip to the bottom if you like
Undead: +1 Hit, Gives you ability to be vulnerable to Holy Light, and the benefit of Unholy Light.
-Note that the Holy Light power is stronger than the Unholy Light power due to the fact that Holy Light leaves the undead player open to character death, and that since the unholy warrior is concentrating on the undead powers, he won't have access Unholy Light later to much later (3 levels of experience if not taking Energy Drain or Life Drain) Also note that with the availability of items and with the yen factor system that Yb is using (which I feel is a little too much and have downscaled in my dungeons myself when DMing), most players won't buy or even consider taking toughness IMO.
Energy Drain: -1 to damage on attacks that deal more than 1 damage.
-This would cancel out holy strike, although, as above, it would take another level for the unholy warrior to get unholy strike, perhaps if this is too powerful, make Holy Strike: +1 damage to Unholy Warriors and Undead, so PC undead without energy drain would receive 3 damage, or 2 damage with Energy Drain.
Life Drain: +1 damage to all attacks.
I agree that this ability is very powerful, although by the undead player gains this, the minimum level to gain it is level 6, and that's assuming you concentrate on nothing else, don't purchase a modifier, and gain 1 extra saved Exp to spend throughout the 5 levels, otherwise its level 6 without concentrating on anything else. At this time, a holy warrior has access to holy light and even more Ki or Holy Strike if they concentrated fully on the holy warrior/ki route. From this point onwards, the undead player is free to pick out whatever abilities, including unholy battery, but won't have access to Unholy Light for a long time...
Taking Holy Light to account: Holy Light is very powerful vs. the undead, especially multiple undead, look at last nights fight with Monderefal, he failed his first holy light but managed to use it again the next round. The reason why it's very powerful vs. multiple undead is that if just one of them gets a 'bad' move and the holy warrior gets a 'decent' move but not quite as good sounding as the other undead monster's moves, they all still take 3 or 4 hits of damage (depending if they have Energy Drain or not), this makes it hard to throw lower CR undead en masse at them (without them having plenty of dirty tricks and shoves), which are *usually* more common than running into a holy warrior or unholy warrior.
I agree Holy Warriors need some powers to balance it, Kalanyr has talked about a possible 'Angel' path, and I suggested one of the final abilities lower all their holy battery costs by -1 (minimum of 2, can't be having a billion shoves...), which might be a good epic power for holy warriors (Ki Mastery?), also add Chi Strike to Holy Battery (kinda makes sense), and perhaps lower the cost of Purity of Focus to 3 (though this makes it easier for holy warriors to use holy light...), but that's just my opinion.
Also, if Life Drain is too powerful overall, and would still be PC usable, maybe change it to a 1/fight power like, Life Drain deals +2 damage and the Undead player heals 2 hits if successful or something similar.
