There's basically no way to just instantly kill someone by biting their head off.
Grabbing part is obviously a grapple, or having the grab special quality. But there is no rules support, as far as I'm aware, for biting someone's head off.
Which isn't to say it can't be done, but if you're my player I would tell you that you can do it as a full round action (that provokes) if the creatures is helpless, and allow for a fort save to resist having their head actually be bitten. Tough spine or something like that.
So in other words, a coup de grace.
WRONG! THERE IS A WAY!
Well... at least there is a way to insta-kill someone with your bite attack, on the second round, or on the round after making an attack of opportunity against a creature. And - as a bonus - you will have a mouth big enough to fit most humanoid races' heads inside.
Just be a permanently enlarged Goblin with levels in Feral Gnasher Archetyped Barbarian.
With Feral Gnashers, if you have the racial trait "hard head, big teeth" you will have a 1d8 bite attack that adds 1 1/2 Strength Mod (as its your only natural attack), and by level 3 you will also gain the GRAB ability.
With any successful Bite attack you get a free grapple attempt with you bite.
With both Improved & Greater Grapple, you can maintain the grapple as a move action - giving your prey the pinned condition, and thereby leaving your standard action to deal a Coup de Grace thanks to the little gem of a feat called THROAT SLICER So, given Greater Grapple's BAB requirement, this takes 7 levels to pull off.
by level 10 of Feral Gnasher, your bite increases to 1d10, and does x3 on a critical hit (i.e. your Coup de Grace) but given that the save vs a coup de grace is (10 + damage dealt), it is unlikely anything was surviving the insta-kill attempts between levels 7 and 10 anyway.
Some other alternatives are to be a human with Racial Heritage: Goblin taken at 1st level, and then multiclass between Feral Gnasher and Lore Warden. Here you would still get the bite attack, but you would also start getting large buffs to your CMB (via Maneiuver Mastery & Know they Enemy) as well as to your CMD to resist grapples and trip (so long as you took the human FCB for fighter) The lack of rage rounds isnt even that big of a deal, as the bite attack and grab ability are not based on rage (unlike the Bite attack from the Animal Fury Rage Power)
Feats: The Cunning Caster, Guild Emissary, Subtle Devices, Throat Slicer, and Underworld Connections feats are not legal for play.
Not legal for society play, though.
Since it is far too good.
The closest you're going to get is a Vorpal Amulet of Mighty Fists (bite attacks are slashing, so it works) and a natural 20, but I don't think you can because you can't normally apply vorpal to unarmed strikes, which is what Amulet of Mighty Fists requires.If your unarmed strikes can deal slashing damage (which can be done through various methods), why could you not apply Vorpal to them?
Because vorpal specifically requires a slashing weapon, which an amulet is not.
It's similar to why the Brawling enhancement can't be applied to bracers of armor, because brawling requires light armor specifically to qualify, not even mitrhal medium armor qualifies.
To qualify for a weapon enchantment the target weapon has to be valid 'on it's own' not become valid through a characters abilities.
Lets say you have a feat called Slicing Mace, which lets you do slashing damage with a mace. A mace still can't be enchanted with vorpal (even if you do the enchanting) because the mace doesn't qualify. When you weild it you can do slashing damage, but the mace itself is not a slashing weapon.
The same thing applies to amulet of natural armor and unarmed strikes.
can a huge creature make two grapples on the same target? like a dragon grabbing your pally then bite grappling the half that is exposed then pulling to tear the guy in half and swallowing the part that is in the maw of the dragon. it would be about the same right?
No. Pathfinder as a system does not support this kind of stuff.
While the mechanics of Pathfinder don't technically support that kind of thing, there's usually narrative wiggle room for a GM with the "rule of cool" (assuming a home game). I've had GMs that allow me to describe things like killing blows, especially if it's a critical or massive overkill, however I'd like. So if you get a beefed up bite attack and you finish off someone in a grapple with it, then you could potentially get away with saying you bit its head off.