Biter is a blackened steel axe (I used battle axe, but hand/throwing axe would work very nicely as well). The axe blade has jagged, sharpened 'teeth', similar to a saw but shaped more like the teeth of a carnivore. Carved straight through the blade are a pair of slots which give the appearance of eyes above the single row of teeth. While these 'eyes' normally only serve as a decorative function, however, if a weapon point (such as a sword point) got caught in one of the eyes, a quick twist of the haft would either break the trapped weapon or disarm the wielder. The handle is tightly bound in a strong black leather of an indeterminate origin.
Biter was created by an extinct and 'advanced' barbarian tribe from hundreds of years ago which culture and secrets are all but lost in the wake of time. It is said that Biter was made using the soul of an ogre chieftain as punishment for making war against the barbarians.
Indeed, Biter has an intelligence. It only communicates with it's owner through a limited empathy. Only a person with psionics or a really weak will, will understand Biter right from touch. The main thing Biter communicates with it's owner is it's hunger for flesh. If the owner feeds it well, it will serve well. If not fed well, well... look at the drawbacks section. To feed Biter, all one needs to do is sink it's teeth into some meat, smoking and being telekinetically pulled towards the blade, the meat will slowly disappear.
Passive – if Biter likes the 'food' it's master is fighting, it will operate as a +2 weapon and deal decent extra damage, which counts as acid damage.
Passive – if Biter is hungry and the 'food' it's master is fighting is acceptable, it will operate as a +1 weapon, with a small extra damage counting as acid damage.
Active – if Biter is happy (well fed), it may heal it's owner in and only in combat. After it's owner is healed, Biter will become ravenously hungry and eventually down right pissed off. Biter chooses when to do this, but may do it by request if the relationship with it's owner has been long and beneficial. It will never do this more than once per day.
Active – if Biter is constantly being blocked or is being used against something non-fleshy, (something it hates) it may induce a 'barbarian rage' in it's master. This might also be a drawback, if it's owner were using Biter to chop down a tree.
Active – on rare occasion, if the relationship with it's master has been really good, Biter might 'leap' to it's master's defence, if it's master is incapacitated or disarmed (see drawbacks for more on the leap).
Most new owner's will suffer from these drawbacks, until they figure Biter out.
Passive – if Biter hates the 'food' it's master is fighting. It will operate as a -1 weapon and usually deals minimum damage.
If hungry, Biter can telekinetically lift itself up and hurl itself at something it wants to eat. Biter will typically not do this more than once or twice a day. Usually picking a critical moment. New owners of Biter usually find their pet/mount is either dead in the morning or scared off and missing. When attacking on it's own, it acts as though it was being used by... perhaps a top notch ogre... hmmm.
If restrained constantly and unfed, Biter will 'hate' it's owner and use it's 'telekinetic leap' to attack it's owner whenever possible. It will constantly act as the cursed weapon above (#1 drawback) in it's owner's hands and it may just leap into the hands of an enemy and function as the +2 weapon above (#1 beneficial) while fighting it's former owner.
Calibrating Biter -
You've probably noticed that Biter has favorite foods and foods he doesn't like. To determine what foods Biter likes depends on your campaign world. Are you the kind of GM who has forty different subspecies of elves? If so, you might want to choose (perhaps randomly) about ten subspecies of the humanoid races, which Biter loves to eat, and also about ten that he hates. Obviously the vast majority of races/subspecies should be on it's neutral list (meaning Biter will eat it if it's hungry but never offers the better bonuses to do so). Scale this number down if you use less. Biter never likes carrion or undead, and always likes animals/humanoids with hooves.
If Biter is the cursed soul of an ogre chieftain, there must be some action on his part that will break the curse/enchantment. Here's some options:
He must eat/kill one evil being of each different race.
He must avenge the barbarian tribe he harmed. This can still be done as the entity responsible for the barbarian tribe extinction is immortal and still around.
Also you might want to change his favourites/hated list once in a while to keep your players guessing.
Final Thought -
Remember that Biter has an intelligence. Most of the above mentioned actions/moods, would be typical behaviours on his part but they do not represent the whole. Therefore, feel free to modify and add stuff. For example if he is buried in an enemy and he's hungry he might not want to come out when his owner tries to pull him free. Mix it up. On the bright side, you only need to imply a personality from him as it's only an emotional being. Also, if the PC's aren't careful they could tick him off right away and never realize this weapon's potential. Maybe hint dropping, or letting the PC's enjoy some of the benefits first, before dealing with drawbacks would be in your best interest.