Fell asleep writing/researching this post last night, parts of it are redundant with Bphill's post and handbook link (because it's seriously a great resource and one I spent a lot of time last night combing, heh).

I think you can technically enter Fiend of Possession as an evil-aligned Neraph (from the Planar Handbook) using a ritual in Savage Species called the Ritual of Alignment to gain the (Evil) subtype, but it costs 56,000 gp and a few thousand xp, which you can't afford until level 11 without breaking Wealth by Level somehow. It's the only LA+0 way to get the Outsider subtype I know of, though, other than the Otherworldly feat. Still need the ritual either way though....

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And, well... here is. a build.

It contains nearly lethal levels of cheese, but I got inspired while digging around some other Fiend of Possession threads.

Start as whatever race you'd like. No, really.

Use the Celestially-Attended Birth background to take the Otherworldly feat at level 1. Now you're an Outsider (Native) with all the benefits thereof.

Next, you'll go on some adventures. A lot of adventures, actually. Play a Psion (Telepath) or an Ardent. Or a Warlock 1/Sorcerer 1/Hexblade 2/Warlock +1/Mindbender X if you're not going Thrallherd.

Take a level of Mindbender, if you're allowed to get into it using the Psionics adaptation (or using Practiced Spellcaster if you're the Warlock/Hexblade), and some Thrallherd levels because why not. Make sure you have your copy of the Player's Handbook II handy when you get to level 13~ish.

When you have enough gold to buy the Ritual of Alignment from Savage Species, do that. Try to get your hands on a Thoughtbottle, as well, and use it to only pay 500 XP for the Ritual... But don't lose it, we're going to need it again in a minute.

Next is the real cheese. Open your copy of the Player's Handbook II to page 197 and very carefully read the rules for retraining class levels. Then read the rules for retraining race on page 198. Then go to page 199 and start making Knowledge checks to learn the location of the Gates of Dawn. When you get there, mow down the EL 8-9 encounter(s), and store your experience in the Thoughtbottle.

Enter the Gate, and instantly retrain (1) your Race, to Dvati, and (2) 3 of your class levels to Fiend of Possession, which you qualify for as an Outsider (Native, Evil). Buy off your new LA by paying a heap of XP (enough to go from 12 to 11). Oops, you're unconscious for an hour, but no big deal. When you wake up, use the Thoughtbottle again to restore your lost experience. Now you can enter the Gate of Dawn again and retrain some more class levels.

You should now be a Dvati Psion 5/Mindbender 1/Thrallherd 1/Fiend of Possession 5 at ECL 12 (assuming you started at ECL 13), around 1000 xp away from level 13. If you wanted to go whole hog Fiend of Possession, you took Leadership or Undead Leadership at 6 instead of Thrallherd, so retrain your class levels to something like Warlock 2/Hexblade 1/Fiend of Possession 6/Legacy Champion 3, then take Legacy Champion to 10 and finish with Hexblade +1 to get +14 equivalent weapons when you possess them and your Charisma to saves and such (either need to retrain your ability scores or use a Charisma based class while leveling or something). Invocations from Warlock have somatic components, so those are a no-go, but Eldritch Blast itself isn't, and neither is your at-will Detect Magic, and since using (Sp) abilities isn't spellcasting you can use Eldritch Blast with both twins each round if you want to. Shame it's only 1d6, but it's better than nothing. You also get the fairly rare luck bonus on saves from Dark One's Own Luck, by leaving the object each day and using it on yourself before repossessing the objects, and it stacks with your Charisma bonus to saves from Hexblade making it very, very difficult to remove you from stuff with spells.

So what does this character actually do? Well, you can do all sorts of stuff with it.

Use one Dvati Twin to possess, then animate a suit of Full Plate and try to get someone to argue that you don't benefit from the armor's enchantments while you are the armor, and the other to possess the Greatsword while they also try to argue that an animated suit of armor can't wield weapons (it probably can).

Or have your cohort/thrall use your possessed armor and weapons, or use what's left of your wealth by level (you don't need it anymore, heh) enchanting a suit of armor for your cohort and have one of the Dvati Twins possess the cohort once you get Fiend of Possession 6.

If you took Ardent instead of Psion, Practiced Manifester can actually keep your Psionic powers fairly relevant (18th level manifesting if you return to Ardent or Thrallherd after Fiend of Possession, though you have "only" +6 worth of bonuses on your stuff then... You can technically advance both with bloodline levels but that's a whole other load of cheese). Since Psionics are mental-only actions you have free reign to manifest powers from the relative safety of the objects you're possessing.

You could also try to possess and animate a noncontinuous object like a bunch of desert sand and try to use that to move the armor, there's not really any RAW for that, though. I guess it isn't much different from animating a statue you put armor on, players try stuff like that all the time.

Of course, you could also handwave all of the Char Op stuff and just build a Psionic Gish (something like Necropolitan Ardent 6/Slayer 1/Mindbender 1/Thrallherd 2/Slayer +9/Ardent +1 with Practiced Manifester) and use Thrallherd to attract a Dvati Warlock 2/Hexblade 2/Fiend of Possession 1 that already paid for the Ritual of Alignment but suffered a bunch of permanent negative levels so you can get it as early as possible. That might get even more books thrown at you, but as a Thrall you can get it at your ECL 6 instead of having to wait until your ECL 13 and jumping through a bunch of hoops yourself.

In any case, you might want to find a real life "breastplate of book dodging" if you try to bring this to a normal DnD table that isn't used to this kind of zany stuff.

And if they are, you could just explain what you want to do and ask to play a Dvati Warlock/Hexblade/Fiend of Possession who used to be a powerful Warlock and got the Ritual of Alignment performed but got permanently level drained by a Succubus/Incubus/Undead creature or something. I'd probably be cool with it at my table assuming the other players were.