Eating a Barghest (it makes sense, I swear!)

[GreatWyrmGold]

I've been playing Rise of the Runelords, and our DM is stepping down for various reasons...one of the big ones being that we're not a fan of his DMing style, in part because he doesn't read the adventure path very clearly. For instance, early on he didn't realize that Malfeshnekor was magically bound to his chamber. We lost three characters there while the rest of us skedaddled. (Though one or two of them were from wanting to replace the character.)
I'm going to be taking over, and I have this idea for a quick adventure that I can play to de-rust my DM skills. Basically, Vorka, the cannibal goblin mentioned in passing, winds up in the service of Malfeshnekor and the two of them create nasty monsters. During the boss battle, after Vorka and Malfeshknor are pushed to a certain point, I had the idea that Vorka could eat/absorb at least part of Malfeshknor, becoming more powerful and giving the PCs temporary mutations.

My question is, what kind of class/abilities could I give pre-transformation Vorka to make this transformation more plausible? Some kind of divine-ey or occult-ey class which leeches off other creatures abilities? Some feat or magic item or template which lets them drain energy? Something? I know that I'm the DM, I can do what I want, but I don't want this to come completely out of left field. I want the players to be surprised, but I want it to be plausible.

[Xuldarinar]

Flesheater barbarian (http://www.d20pfsrd.com/classes/core-classes/barbarian/archetypes/paizo-barbarian-archetypes/flesheater-barbarian-archetype/) stands out most to me. Take a look at it. I don't think it is that huge a leap from eating things like magical beasts for power and someone turning to consume a barghest all things considered.

[DrMotives]

There actually is a planetouched race that's a goblin with barghest ancestry that sits at LA +1 or +2 somewhere, can't recall exactly where the thing is. Seems like it'd be worth looking into for you though. You also might look at druidic avenger from Unearthed Arcana, which is the druid variant that gains barbarian rage at the cost of an animal companion. Since it leaves wild shape alone, you could stack it with other druid variants that tweak that ability to get something more in line with a barghest's shapeshifting.

[Xuldarinar]

There actually is a planetouched race that's a goblin with barghest ancestry that sits at LA +1 or +2 somewhere, can't recall exactly where the thing is. Seems like it'd be worth looking into for you though. You also might look at druidic avenger from Unearthed Arcana, which is the druid variant that gains barbarian rage at the cost of an animal companion. Since it leaves wild shape alone, you could stack it with other druid variants that tweak that ability to get something more in line with a barghest's shapeshifting.

Wrong game system (This is pathfinder, not 3.5, or 3e in the case of the worghest), so there would need to be some substantial conversions to bring onto par. That said, it is interesting you bring those up. The druidic avenger does have some equivalents in pathfinder, though none that I know of that grants rage to a druid: One can argue the entire shifter class, but also there is the mooncursed barbarian. The ex-druid's planar extremist also does that, in its own way.