Artifact by default... not necessarily. Hadryllis from Forgotten Realms is a named magical item of "some potency*." It's not an artifact, just an intelligent +3 outsider bane holy cold iron greatsword.
That said, I DO want it to be near-artifact in status. It was "given" to Sigmund by Odin, after all. This is what just popped into mind as I was rereading the Saga of the Volsungs:
Gram: Adamantine +5 Berserker Dragon Bane Greatsword. In addition, a wielder can formally claim Gram as his own by grasping the hilt and as a standard action willfully expending 1,000 XP. At any point thereafter, the wielder may call upon a surge of fortune as an immediate action, treating his next attack roll with Gram as a natural 20. (The attack roll must occur within one round of invoking this power, and you must still roll to confirm the critical hit normally). No one else can formally claim Gram until the wielder expends the surge of fortune, or he dies, breaking his link with Gram.
I'll probably tweak it a bit yet, but it's basically just a "normal" magic sword, plus an XP-requiring ability for the wielder to cast Surge of Fortune (Comp Champ) or Limited Wish on himself (recall that Limited Wish can "Produce any other effect whose power level is in line with the above effects, such as a single creature automatically hitting on its next attack...")
One might even be able to interpret it as preventing the application of a Bless Weapon effect, as the nat 20 ability is coming from the sword...
Alternatively, or additionally, I could see allowing a once-per-lifetime Surge of Fortune/Bless Weapon combo (after all, an archivist or a cleric with some Oil of Bless Weapon could do this several times a day.)
Not that it really matters. The PC I expect to wield this can cast True Strike. You can guarantee he'd be putting all the bonuses onto the confirmation roll that he can.
The XP cost is just a thumbnail estimate, more expensive than Limited Wish's 300 xp cost because it allows anyone, even people who couldn't normally cast a limited wish, to gain/control the benefit.
*It has the unique property of being able to shatter the Adamantine Binding which tethers Eltab and the scattered demoncyst pockets of the 248th layer of the Abyss to the Demonlands of Faerun.