Daemons!

[Parakus]

And no, not the ones with glowing eyes eyes and black wings. These daemons are more akin to the ones in the His Dark Materials trilogy (if you haven't read it, in the series, everyone in the book has a lifelong companion, in the form of animal, that they have a bond with (ie, if the daemon is injured, the human feels the pain, and vice-versa)). Anyway, I thought it would be interesting if dnd characters each had a daemon. The character's class might affect the daemon's animal form. Also, in the books, if a human travels too far from his or her daemon, they suffer excruciating pain and possibly death.

So, with my huge paragraph of background in mind, any thoughts?

[TheLogman]

Familiars and Animal Companions are both limited forms of what you're looking for, albeit without the shapechanging.

I can easily see an Animal Companion (Like a Druid's or Ranger's), with the Shapechanging ability you want, and the drawback being the shared pain (Non-Lethal Damage when partner takes damage) and the death clause.

[wadledo]

You might want to look at the Changeling Wizard Sub levels.
I think level five has something almost exactly liek what your looking for, mechancialy, though no shared pain.

[BlueWizard]

Yugoloths?

[Tiki Snakes]

Use the ranger pets as a base (not read up on familiars yet, admittedly). Share a standard, move and minor action a round between them both, with them able to both move as part of the same move action (however it described it in martial power. Eh.)

But as it's intrinsically part of the person, basically an external soul, have the two share a single HP total, and so on, also. (Might be best to consider them one creature for the purpose of area attacks that hit both, also, otherwise bursts and blasts become over-powered quickly.

I'd say that ideally, you wouldn't limit the type of Daemon by class or even suggest a link at all. The Daemon (which if you're doing it dark-materials style would be pretty much stuck in a single form by then, of course) is a reflection of the person's character and general nature. The best way to do that, I think, would be to have it entirely up to the PC.

there are two ways you could do damage. My instinct would be that an attack from the daemon counts as an unarmed attack, just as if you had made it yourself. Altenatively, depending on the type of creature it emulates, the Daemon may have more, or less. Or you could more closely follow the ranger and wizard routes, (again depending on the creature) and have any number of extra stuff, like proper damage, skill bonuses and so on.

The main bonus would, assuming the former, be that of essentially getting to make an extra perception/insight check, and similar. ('Lyra, look out!) probably using the characters stats, but whatever is good. Of course, you'd have to be careful with how you ruled on Combat advantage. If the character can use his daemon to flank, that has suddenly made the Rogue's life almost unfairly easy...

[Yuki Akuma]

Unless all the PCs are under twelve, the daemons wouldn't be shapechanging any time soon.