When a character becomes a 10th level Alienist (prestige class), the following change is said to occur (emphasis mine):
Alien Transcendence (Su): A 10th-level alienist, because of long association with alien entities and intense study of insane secrets, transcends her mortal form and becomes an alien creature. Her type changes to outsider. Additionally, she gains damage reduction 10/magic and resistance to acid 10 and electricity 10. Upon achieving alien transcendence, an alienist undergoes a minor physical change, usually growing a small tentacle or other strange feature, such as an extra appendage, organ, eye, or enigmatic lump. An alienist can hide this abnormality in a robe or hood, but the alien growth is not under the alienist's control and sometimes moves, twitches, opens, or otherwise animates of its own accord. This applies a —4 penalty on Disguise checks an alienist makes to conceal her true nature. Anyone who shares an alienist's predilection for study of the Far Realms immediately recognizes her transcendent nature, and she gains a +2 circumstance bonus on all Charisma- based skill checks and ability checks when interacting with such beings. She gains a +2 circumstance bonus on Intimidate checks involving any other creatures to whom she reveals her abnormal nature.
So the character becomes an outsider. d20srd describes outsiders as such:
An outsider has the following features.
- 8-sided Hit Dice.
- Base attack bonus equal to total Hit Dice (as fighter).
- Good Fortitude, Reflex, and Will saves.
- Skill points equal to (8 + Int modifier, minimum 1) per Hit Die, with quadruple skill points for the first Hit Die.
An outsider possesses the following traits (unless otherwise noted in a creature’s entry).
- Darkvision out to 60 feet.
- Unlike most other living creatures, an outsider does not have a dual nature—its soul and body form one unit. When an outsider is slain, no soul is set loose. Spells that restore souls to their bodies, such as raise dead, reincarnate, and resurrection, don’t work on an outsider. It takes a different magical effect, such as limited wish, wish, miracle, or true resurrection to restore it to life. An outsider with the native subtype can be raised, reincarnated, or resurrected just as other living creatures can be.
- Proficient with all simple and martial weapons and any weapons mentioned in its entry.
- Proficient with whatever type of armor (light, medium, or heavy) it is described as wearing, as well as all lighter types. Outsiders not indicated as wearing armor are not proficient with armor. Outsiders are proficient with shields if they are proficient with any form of armor.
- Outsiders breathe, but do not need to eat or sleep (although they can do so if they wish). Native outsiders breathe, eat, and sleep.
For one, I would tend to believe the character is a native outsider at this point, because it not only has ancestors in the material plane, it was born on it. Am I right?
Now, more importantly, what benefits does the alienist gain from becoming an outsider? Here are a few that they could possibly gain:
- More HP. If so, are all Hit Dice from previous character levels rerolled as d8, or is it only from that level onward?
- The same base attack bonus as the Fighter's, along with the extra attacks that come with it. If so, is this for total character levels or only from this level onward?
- Improved saving throws (if so, by how much?)
- More skill points (8 + int instead of 2 + int). If so, does the character get skill points from previous levels or is it from that level onward?
- Darkvision (60 ft / 12 spaces)
- Impossible to resurrect unless using true resurrection, wish or miracle if not a native outsider (see above)
- Proficiency with all simple and martial weapons
- No need to eat or sleep if not a native outsider (see above)