Recently JohnnyIrish sent me a letter asking for some advice and clarification for playing undead characters. Since I covered a lot of points in my response, I figured it might be helpful to post it here for anyone else who was curious about playing an undead character.
After our talk the other day in openrpg I'm seriously considering being a necropolitan too now. I have a similar but obviously different story (I imagine the reasons to become necropolitans ie to escape death would be the main reason to become one).
Not a problem, let me see if I can help get you on your way.
Some necropolitan questions though:
1)When ones becomes undead do they loose their human traits? (extra skill point/lvl and extra feat)
It depends on which type of undead creature you turn into. The necropolitan and damned (HoA version of the necropolitan) retain their base racial abilities, such as an elf's low-light vision and a human's bonus feats and skill points. Other types of undead, such as ghouls, ghasts, mummies, and so forth generally do not unless you are allowed to use the templates released in Libris Mortis and Savage Species, in which case I believe they are retained. Really, it's a matter of what game you are in, but the default case with such undead is they are entirely new creatures.
2)It says their HD become 1D12 but that's for racian HD no? So you'd still get 1d4 hp for being a mage no?
In 3.x, undead use the d12 HD regardless of where their HD comes from, which is odd to say the least, but they have no Constitution modifier. In Pathfinder, undead use the hit die for their class, with any racial hit dice being d8s, and apply their Charisma modifier to their HP. For example, my undead PC Elissa has a level of Wizard (Pathfinder). Her base hit dice is 1d6.
3) Your using pathfinder rules for cha to hp for undead is more then just a class or race. Did you get dm permission for it by any chance?
Actually it is part of the undead's type in Pathfinder. The undead race in Pathfinder has 3/4 BAB, d8 HD, and a series of immunities and abilities. They get Charisma to HP and Fortitude, but are no longer immune to critical hits or sneak attacks. This is primarily a balance change since undead were generally too hearty at low levels and too frail at high levels (d12 HD is pretty huge at low levels, but you need modifiers at high levels). Since it is a feature of the race itself, I wasn't aware that it was something that had to be requested, merely noted.
A few things you will probably wish to keep in mind if you plan to play an undead character.
1) Undead are fragile. Undead heal via negative energy which means you generally will have to provide your own healing for adventures, as you generally cannot benefit from allies who have healing capabilities, and you usually cannot confiscate healing potions from your enemies, which means you'll probably need to invest in items to heal with negative energy, and be extra careful.
2) Undead creatures die at 0 HP. While we get our Charisma in place of Con for most things, we have no negative Hp threshold like a creature with a constitution score. Thus, while most PCs have between 5-20 extra hit points from when they hit 0 before they actually die, undead characters do not. This generally means that to survive to higher levels you will need to play very defensively (notice I emphasized Elissa's Charisma as hard as possible despite being a wizard and have Toughness). Undead that begin with racial hit dice such as ghouls, ghasts, and mummies are less affected by this, but it's still a pretty big deal. This is especially true since undead are no longer immune to critical hits.
3) Closely connected to 1 & 2, undead are a pain to raise. In core there is no good way to raise an undead character back from being killed barring certain very high level magics such as resurrection and wish type spells. Spells like raise dead just won't cut it. This means that death holds a much larger impact for undead characters, because most PCs can get carted off, have a combination of raise dead and restoration cast for them and be good as new. Undead characters do not have this luxury.
There is a spell in Libris Mortis that allows you to raise an undead creature, but it's a 6th level spell, requires the body to not be destroyed via Turn Undead, and must be revived within 1 day per caster level. Additionally you permanently lose a level, HD, or 2 points of Charisma which cannot be recovered by any effect. It's possible that since spells like raise dead were lessened in effect, this spell would probably receive a similar boost, but I'm not certain exactly how that would work (Pathfinder sticks you with a negative level until you get a restoration spell). I would imagine it would be a similar effect, possibly requiring additional spells to remove the penalties for being revived.
The primary benefits of being undead in Pathfinder are very similar to the benefits of being a Warforged. You don't eat, drink, or sleep, and you are immune to most effects that require fortitude saves such as poison, nausea, and so forth. Undead also enjoy immunity to mind influencing effects but lose the ability to be targeted by a number of beneficial spells such as enlarge person, reduce person, and bonuses such as heroism. Also, undead gain a number of new weaknesses. They become vulnerable to effects such as command undead, hide from undead, hold undead, control undead, a cleric's turn and command undead, as well as being set up for spells like searing light to be more powerful against. Disruption weapons are particularly dangerous, as is the Pathfinder cleric spell disruption weapon which turns a weapon into a disrupting weapon using the cleric's save DC (fortunately you're only vulnerable if the cleric is equal or higher level but it's still dangerous). Unfortunately for undead, while the normal versions of most of these spells have effective counters, most of these spells lack anything that can counter them (hide from undead arguably isn't even vulnerable to true seeing). The most effective method I've found for preventing most of these spells is via spell turning and spell immunity, but it's a bit difficult to set up those defenses (especially early on).
Hopefully this should get your started. If you plan to be an undead, make sure to be wary of their drawbacks. Most people see undead characters as a bundle of immunities and don't consider their drawbacks until it is too late. If you're looking for an excellent undead to play to be a gish, I would recommend a Ghast, because of their impressive ability modifiers to most everything and their natural attacks and stench (but I would recommend the Civilized Ghoulishness Pathfinder feat to pass for human and suppress your stench. Alternatively for a warrior, Mummies are very attractive due to their +14 Str, +10 natural armor, and DR 5/-, but they have fire vulnerability and are really slow (20 ft base speed), so I'd recommend some boots of striding and a ring of energy resistance (fire) as soon as possible.
Please take care, and hope to game with you soon. ^-^