What happens when...

[Werephilosopher]

...a hound archon and a succubus undergo fusion (http://www.d20srd.org/srd/psionic/powers/fusion.htm)? Specifically, what happens to their subtypes? Does the fused being have the Good, Evil, Law, and Chaos subtypes, or do they cancel out? If they keep them all, what does that mean for spells/attacks/etc. that have greater effect on one subtype but reduced effect on the opposite subtype?

[John Longarrow]

You get one UGLY girlfriend? (girlfiend??)

[Kazudo]

It doesn't specifically say anything about their subtypes. I have no reason to doubt that two epic level creatures fusing into one (pretty beastly) epic level character would be anything but all subtypes at once.

It would get pretty busy in that head of theirs.

[Lightlawbliss]

...a hound archon and a succubus undergo fusion (http://www.d20srd.org/srd/psionic/powers/fusion.htm)? Specifically, what happens to their subtypes? Does the fused being have the Good, Evil, Law, and Chaos subtypes, or do they cancel out? If they keep them all, what does that mean for spells/attacks/etc. that have greater effect on one subtype but reduced effect on the opposite subtype?

first: the willing part is unlikely at best
second: by RAW it would have all the types (if you treat it as a racial ability), but a DM would be fully justified in saying they go away or that it depends on who is in control.
third: that would be an insanely high level being combining with it's polar opposite instead of with, say, a solar

[Lord of Shadows]

First, in a tip of the hat to Robin:

Holy Melting Pot..!

Ok. What you would likely end up with would be a very erratic creature, possibly even suffering from bipolar or some other mental condition. I would guess that most of the time the extremes would counteract each other, allowing the creature to exist more or less normally. But heaven help the immediate area if the neurons misfired.

[Alex12]

Per RAW, I'm pretty sure they'd have all 4 subtypes. Much like a Tainted Scholar (with a class ability that gives all their spells the [evil] tag) could cast a [good] spell) and it would then be both [good] and [evil].

[Doc_Maynot]

A Succubitch?

[torrasque666]

A succubutt?

[Muggins]

One is the manifestation of sin, born from the infinite layers of the Abyss to tempt and corrupt mortals.

The other is the manifestation of pride and justice, a dogged warrior who is one century away from retirement and who doesn't play games.

Together, they fight crime. Maritial affairs, mostly, but they're looking to branch out. Here's their number.

[Xerlith]

One is the manifestation of sin, born from the infinite layers of the Abyss to tempt and corrupt mortals.

The other is the manifestation of pride and justice, a dogged warrior who is one century away from retirement and who doesn't play games.

Together, they fight crime. Maritial affairs, mostly, but they're looking to branch out. Here's their number.

This made me earn some weird stares for laughing aloud in the train. Have a cookie. or a cookie jar. :smallbiggrin:

[Werephilosopher]

To be clear, the hypothetical archon and succubus aren't really those, they're an erudite and his psicrystal shapechanged (http://www.d20srd.org/srd/spells/shapechange.htm) into them. So there really wouldn't be conflicts of personality, since one personality is a piece of the other. Also, would a [fire] creature fused with a [cold] creature have immunity to both fire and cold, assuming they kept both subtypes?