You know, once upon a time there has been one called Sarevok Anchev. That would be a costume I'd like to see Xander wearing.
That is, Sarevok Anchev from Baldur's Gate (Released 1998, after "Halloween")
And not the goody-two-shoes-drizzt-wannabe-Chaotic-good Sarevok, but the "I will be the last.... and you will be the first to go *neck snap*" Chaotic Evil one.
If only for Sarevok channeling the essence of a cross-dimensional lord of murder (adn getting into deep manure for doing so) and summoning a nigh-impenetrable armor by doing so.
And, of course, the irrational urge to slay everyone more powerful than him, less powerful than him and not immediately useful to his plans.
After all, Sarevok was honest - he hated everyone.
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Another one would be Ignus, as in "Ignus burnsssssss......", from Planescape: Torment. (release date: 1999, after "Halloween")
The chaotic neutral (read: bloody fucking insane) pyro-mage who got turned into a living conduit to the elemental plane of fire (that means he is CONSTANTLY - ON - FIRE but can't die because of his magic prowress... I think constant immolation fried his brain even further).
The Ignus side of the X-Man would give everyone pause once he gets his hands on anything flammable, not flammable or possibly flammable, because it WOULD light up, and do so quickly.
And, of course, try to sleep if every time you close your eyes you watch yourself burning down a giant hive full of civilians just because you COULD do it. Nuh-uh.
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And, one more: Diablo, from said game(s). (Release date: 1996, before the Halloween episode)
Pro's: Big. Red. Awesome. Totally kick-ass.
Con's: Your best friend wants to slay you. You have the irrational urge to corrupt the mayor, his son(s) and the local priest(s). You can't stand any kind of crystals around you for long. They tend to.... break.
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Yes, two out of the three are post-HW, but they're all pretty damn old, and, seriously screwed up, beyond the simple "bam instant power". Especially Ignus - because unlike Ignus, Xander is NOT imbued with the magic necessary to survive having the raw stuff of the elemental plane of fire course through him for long.