Deamon personality (3.5e Flaw)
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 Demon Personality
Summary: A demon lurks deep in your psyche, but is desperate to fight its way out.
Effect: Whenever you are hit in combat, or in some other particularly stressful situation, the demon that lurks in your mind sees an opening in your mental barriers. If this happens, you and the demon fight for control over your body, and you must make a will save to maintain control. Simultaneously, roll for the demon, which has saves as you would but without your ability modifier. Which ever comes out on top takes control. If your will save is higher, you carry on as normal, although you are aware that the beast within just attempted to take control of your own body from you. The demon can only do this once a day (game time), but can occur more often if you take a critical hit or if the player is under any considerable mental stress. This latter ruling should be discussed between the DM and the player as to exactly when these times occur
If the demon succeeds, however, it takes over your body, trapping you while you can do all but watch while a raging monster turns to whoever is closest to you (ally or enemy) and begins to attack them, and your eyes turn completely black, with wispy shadows rising from them like smoke. The demon should taunt your allies while it attacks them. Mechanically, a number of changes come over you, but only for the duration of the demon's control:
- Rage: This is equivalent to the Barbarian's power - your demon-possessed body temporarily gains a +4 bonus to Strength, a +4 bonus to Constitution, and a +2 morale bonus on Will saves against external forces, but takes a –2 penalty to Armor Class. The increase in Constitution increases the demon’s hit points by 2 points per level, but these hit points go away at the end of the rage when your Constitution score drops back to normal. (These extra hit points are not lost first the way temporary hit points are.) While raging, your demon-possessed body cannot use any Charisma-, Dexterity-, or Intelligence-based skills (except for Balance, Escape Artist, Intimidate, and Ride), the Concentration skill, or any abilities that require patience or concentration, nor can the demon cast spells or activate magic items that require a command word, a spell trigger (such as a wand), or spell completion (such as a scroll) to function. The demon can use any feat it has except Combat Expertise, item creation feats, and metamagic feats. However, unlike the Barbarian's rage, the demon's rage is limited only by the length of its activity (before you take control again).
- Your alignment for all extents and purposes changes to Chaotic Evil, so Smite Evil will do extra damage, and Detect Evil will show your alignment as Chaotic Evil, etc.
- Every round, you can repeat the opposing will saves for a chance to take control of your own body once more. However, the demon has a +8 bonus to its will save on the first round, which cumulatively gets halves each round - so it's +8 on the first round, +4 on the second, +2 on the third, +1 on the fourth and no bonus on the fifth. Removing the demon from control knocks you unconscious, just as if you had been dealt non-lethal damage equal to your current HP.
- You gain Smite Good as a supernatural ability for 1/4 of your total level/day, as the Blackguard ability of the same name - the demon adds your Charisma modifier (if positive) to its attack roll and deals 1 extra point of damage per your class level. If the demon accidentally smites a creature that is not good, the smite has no effect but it is still used up for that day.
- You grow temporary +1/per three levels claws, and can attack with them for 1d6 damage per five levels.
- The demon can use any spell from the Evil and Chaos domains, as a cleric of your total level. Any feats you have related to spell-casting cannot be used. Any spells on the two lists which overlap can be used twice.
- Any attacks which previously would have normally affected you still affect the demon-possessed you , as well as any attacks only viable against outsiders.
- A remove curse, dismissal, banishment, wish or miracle or similar spell can restore you to normal. A wish or miracle spell can do this without turning you unconscious.
Once your friends have used non-lethal damage to knock you unconscious, or you have wrested control back from the creature, when you awaken, you will be yourself again. The damage the demon had been dealt has all but disappeared, but any damage you were dealt before becoming possessed remains with you, and the damage done to any relationships is soon sure to sink in.
If you die when the demon is in control, your soul becomes utterly consumed by the demon, making you unable to be brought back to life with anything other than a True Resurrection. Additionally, the demon gains a saving throw against this (yours) and turns into a battle of wills against whoever is resurrecting you. If they lose, the demon jumps over to them. If the demon loses, then you are brought back to life, demon free.
You and your demon can talk to each other but you are the only one who can see/hear him. It looks like you’re talking to yourself. You can hear and see the demon like the Hallucinations (Physical) flaw. And if your demon wants to he can attack you in your dreams like Nocturnal Enuresis (Note: you can get hurt in your dreams but when you wake up the damage is gone. You gain, whoever's in control, a -2 to concentration checks, as it is hard to focus with a demon whispering in your inner ear.
Removing the demon is particularly difficult - with a wish or miracle you and your companions may enter your psyche as psionic projections, and attempt to fight it. The demon should be level-appropriate for the party (so a party full of level 12 characters don't end up fighting a Balor), but over CR 10, or a Succubus (for lower levels). It will have created appropriate defenses as well.
Either you or the DM can play the demon's actions, though if the DM trusts you to play fairly when acting as the demon, it is recommended they let you play, as the demon should know everything you know, but only everything you know.
 Optional Rules (DM's Discretion):
List of flaws that count toward your DC save:
Some flaws can have either positive or negative effects towards your Will saves against the demon. Bear in mind, DMs, that this can unbalance the power quite severely.
- Clinically Depressed Robot flaw -2
- Compulsive Killer flaw -10
- Conceited flaw +3
- Coward flaw -4
- Ghost-Warped flaw -8
- Gullible flaw -10 because the demon can lie of his motives, getting you to believe him.
- Heartless flaw -10
- Hemophobia flaw -1 if within view of blood.
- Incompetent flaw -2
- Lovestruck flaw +5 but in front of the one you love you gain another +5. This stacks.
- Low Pain Threshold flaw -4
- Merciful flaw +10
- Misanthrope flaw -5
- Paranoia flaw +5
- Pride flaw +10
- Short Temper flaw -5
- Stubborn flaw +10
List of demons you can have:
If the DM desires, he can set the type of demon which possesses you. The DM should provide some sort of mechanical change to make this worthwhile.
List of Domains:
If the DM wants, they can assign domains to the demon other than Chaos and Evil to characterize it better.
- Air Domain
- Animal Domain
- Charm Domain
- Darkness Domain
- Death Domain
- Destruction Domain
- Earth Domain
- Fire Domain
- Healing Domain
- Knowledge Domain
- Luck Domain
- Madness Domain
- Magic Domain
- Mind Domain
- Nobility Domain
- Plant Domain
- Protection Domain
- Repose Domain
- Rune Domain
- Scalykind Domain
- Strength Domain
- Sun Domain
- Travel Domain
- Trickery Domain
- War Domain
- Water Domain
- Weather Domain
Also Grants 1 Bonus Feat
DM: this flaw provides a good way to have interactions with demons, if a demon possesses a character. The demon could have a quest of its own that the PCs must discover and thwart, while the demon is in their midst.