This question came up in one of my games.
The players are getting a weapon that is a minor artifact. Aside from doing other stuff, it's a short sword of the planes.
My question is: can it be upgraded via craft magic weapons to add new properties to it, like make it keen, agile, etc.?
If possible, which would be the cost of doing it?
Anything's possible with imagination!
As far as cost goes, there are probably lots of different ways you can adjudicate that. Here's one:
The price of a sword of the planes is 22,310 gp. That puts it at between a +3 (18,000 gp) and a +4 (32,000 gp) weapon. I'd probably treat it as a +3 weapon for upgrades (I'm a benevolent despot GM). So adding keen (for example) would make it a +4 equivalent, which would cost either 5,000 gp (if you added the upgrade cost to the price of the artifact) or 9,000 gp (if you added the upgrade cost to the price of a +3 weapon).
This link in the PRD to Ultimate Campaign may also be helpful.
Note the "caster must be 3X the weapon bonus" rule as well. In this case, that means you need at least a 12th level caster to upgrade this particular sword (if you treat it as a +3 equivalent weapon). Some may object that the base sword requires a CL 15--perfectly reasonable. I wouldn't rule it that way, but that's why GMs exist, instead of algorithms.
EDIT: One caveat - if your campaign spends a lot of time fighting outsiders or on planes other than the material, elemental, Astral, or Ethereal, consider treating this as a +4 weapon (instead of ~+3), since that's how it will behave most of the time.
There aren't explicit rules for it.
As for suggestions.
Whatever the existing enhancement bonus is on the item, just do the difference between the current and the increase. Just like you were upgrading an non-artifact item.
Upgrading a +1 sword to a +2 keen sword is the same as upgrading to a +3 sword. This would cost 16,000 gp (18,000 for +3 -2000 for +1).
For the specific weapon you're looking at, it's difficult to price since the artifacts affect is to change the enhancement bonus based on where you are or what you're attacking. And the enhancement bonus can't exceed +5.
The item is from Skull and Shackles, and I don't think there will be a lot of outsiders, aside from the PC's captain, who is an Undine. The last book is going to have some of them, but nothing to worry about.
The fact that the artifact itself has a changing enhancement bonus is a pain in the ass, but I'll come with something to keep it balanced.
I just needed to make sure that an artifact could be upgraded before allowing it in my game, as it isn't a regular magic item.
Our bard is thinking of using it as a weapon but without some changes it wouldn't benefit her a lot. The other characters are not proficient with short swords or married to their weapons (Inspired blade and Magus with a scimitar).
Also to clarify, the sword of planes isn't an artifact. Though it is a specific magic item.
This one isn't exactly a Sword of the Planes.
It is a minor artifact that acts like a short sword of the planes and has another aditional features.
This question came up in one of my games.
The players are getting a weapon that is a minor artifact. Aside from doing other stuff, it's a short sword of the planes.
My question is: can it be upgraded via craft magic weapons to add new properties to it, like make it keen, agile, etc.?
If possible, which would be the cost of doing it?
Artifacts are magic items created outside of the rules system. There are by definition then, no universal rules answers to that question.
Only if you are using the legendary items rules from Mythic.
If using the mythic rules, the answer is yes.
This one isn't exactly a Sword of the Planes.
It is a minor artifact that acts like a short sword of the planes and has another aditional features.
The problem with upgrading an artifact is that it is completely lacking in the pricing information needed to know the cost of upgrading it. Are the additional powers that it has as an artifact an unknown flat cost or an unknown number of pluses?
Yes, that's the main issue. Even if I am allowing it, I wouldn't know how to price it right.
Being an artifact, my players are kinda excited about having and using it, but as only 1 player can benefit from it, and without any enhancement, he'd not be getting much advantage from using it.
Even if the weapon is not plot related, I can understand that finding such an unique item only to realize it's mostly worthless to them can be kinda annoying, so I'm not against allowing them to upgrade it.