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As voideternal@ notes, Undead and Outsider are both primary types. A creature can only truly be one of those types, and there are very few edge cases where they can be "treated" as more than one primary type (e.g. aasimar with scion of humanity), but those are to my knowledge always alternative racial traits, never baked into the creature.
Your best bet is to grab an existing demon/devil and change its type to Undead while keeping its subtype. This would negate con, give it Cha mod to HP, make it heal from negative energy, etc. Their HP will probably drop somewhat, but they'll gain a host of immunities in exchange.
Undead/Outsider is a dangerous play to make, by the way. Both types individually have a large host of resistances and immunities, and overlapping the two makes for a creature that will be immune and/or heavily resistant to just about everything. There's a reason why Undead and Outsider(evil) are the most common banes and favored enemy types.
Nightshades are the only undead made from fiends I can think of, outside of getting an undead template and putting it on a fiend.
Vampirefy an Outsider. Or another template-based Undead on a Demon.