How to handle an intelligent sword that grows alongside it's wielder?

[MonkeySage]

One of my players in a campaign based on journey to the west was given a bastard sword by his oni mother.

The sword is intelligent(currently has the mind of a sadistic toddler), and grows stronger with the one wielding it. In effect, every few levels, the sword gains a new ability enhancement, based on how it's used. It also grows more intelligent, and it's personality is shaped by how it's used.

The personality that it has now is based on his mother- a sadistic oni.

So I'm wondering how I might handle this in the long run, any neat ideas you guys can come up with?

[HunterOfJello]

There's a whole book on this. It's called Weapons of Legacy.

The examples in the book generally suck and the entire idea of penalizing a character in order to gain what is pretty much already a level-appropriate item is terrible. However, if you removed the penalties to the player that the book suggests and use the rules for creating custom legacy items in the back of the book, then you can make some fun stuff.

[MonkeySage]

The book is interesting, although a thought just occurred to me... is it possible to treat this thing as a character in it's own right instead of as a weapon? It does after all have intelligence.

[Gemini476]

The book is interesting, although a thought just occurred to me... is it possible to treat this thing as a character in it's own right instead of as a weapon? It does after all have intelligence.

Why? Intelligent weapons are already a thing. They're objects that are also characters, I guess.

But yeah, if your PC is going to use it as a weapon you should stat it as an intelligent weapon rather than, say, a familiar.

[Red Fel]

There is a section of the SRD (http://www.d20srd.org/srd/magicItems/intelligentItems.htm) (and by extension, it's in the books as well) governing intelligent items, Ego Scores, and attempts by the item to exert its will over the wielder. Read it, love it.

I would suggest using one of the Weapon of Legacy fixes - the concept proposed by the book is good, but the mechanics are wonky. You can find plenty of proposed fixes - Google is your friend.

I would make it a Legacy item, with it gaining Intelligent Item at its first category, and remodeling its Legacy Rituals to involve either leveling up or accomplishing great heroic feats.

[Firest Kathon]

In Pathfinder there is a class running along exactly this concept, the Bladebound Magus (http://www.d20pfsrd.com/classes/base-classes/magus/archetypes/paizo---magus-archetypes/bladebound) archetype.

[Bronk]

Item Familiars have rules for intelligence, periodic upgrades and adding powers... You could treat it as if the player had that feat, but it isn't under their control and you pick the upgrades...

[ShurikVch]

Dragon #289 have rules for leveling up items.
Instead HD, skill-points, feats and class features, item get enhancement bonus (http://www.d20srd.org/srd/theBasics.htm#enhancementBonus) or Special Abilities

[Particle_Man]

Maybe have a look at the Ancestral Relic feat in Book of Exalted Deeds and reskin it?