Well, I've been pondering writing these up for awhile, but have refrained
because they're rather potent. But they fit so well into the Birthright
setting because of one of their special powers, I just can't resist any
longer.

These swords are inspired by Harry Turtledove's Videssos Cycle, which I
periodically recomend as good stuff for any Birthright gamer to check out.

N.B. I'm just putting in a sketchy history/origin; change it to fit your
own campaign as needed, but because the blade's have been lost for awhile,
they can really show up wherever they are required. Without further
prelude:

Swords of Realm Defence
These twin blades were created by Rjuvik Druids during the time the
Anuirean Empire was spreading into their lands. They were enchanted to
protect the freedom and unique culture of the Rjuvik people, but it has
subsiquently been found that they will provide the same protection to the
lands of any of their wielders, neutrally.
Each sword apears identical, a finely crafted blade with runes running the
entire length of the sword. If Read Magic or Read Languages is cast,
specific results (I.E. a direct translation) will prove impossible to
obtain, because the sword's defence against spellcraft will foil it.
However, the caster will know that they speak of protection, and the
defence of the homeland. Both swords (and their are only TWO in all
Cerilia) have the following powers/special abilities:
In the hands of an unblooded commoner they function as a +2 sword, with
the ability to Detect Magic within a 120 yard radius (magic cast in, or
directed into this range causes the runes to glow with varied intencity; if
the magic is weak and at the edge of the radius, they glow faintly.
Powerful magic cast directly at the wielder causes them to flare
brilliantly.)
It is in the hands of a Blooded Scion or Regent that their true power
shines forth. The wielder must also be Good in alignment (either Lawful,
Neutral, or Chaotic, however). In the hands of such a person the sword is
+3, and in addition to Detecting Magic (as described above), it also offers
powerful protections against magic when held or wielded (I.E. you can grasp
the hilt of the shiethed sword and gain the protection; but just wearing it
on your belt will not transfer the protections described below): The sword
provides 50% Magic Resistance in a 5' radius, and Dispels Magic in a 5'
radius at the level of magic use equal to the experience level of the
wielder. In addition, if the wielder is a Regent, it provides +6 Damage to
dedicated enemies of his Realm (as determined by the DM; shouldn't be
everyone in opposition to the Regent, but just thouse who are fiercely
opposed to the Regent and out for his realm's distruction).
It is when the swords are used together, by wielders who have dedicated
themselves to the defence of a specific ream, that their true power shows.
If wielded by two different Blooded Scions or Regents, and the blades touch
each other at a time and place of danger to the realm, powerful magics will
be released which will act to remove, or possibly even destroy this danger
(area of effect can vary from an individual, to a entire battlefield, at
DM's whim). No Divination spell, Commune, or even a Wish will determine
what the exact effect will be in any given situation; the wielder's cannot
know the outcome until they actually invoke this power (which should keep
them from using it at every opportunity).
Only the DM can judge what will happen. In the first instance the swords
were used in this fashion, both wielders and a large part of a Legion were
transported to another plane. In another instance, an enemy Lich was
shifted to the Grey Wastes, but the army he brought with him to endanger
the wielder's homeland remained on the battlefield and continued to fight.
(DM is encouraged to be creative with determining the effects of this
power. . .and to insure that players only wish to use it in the most
extreme circumstances; they may go wherever their foes go. . .)
N.B.: it is important also that they be wielded by two different people;
if the swords are held one in each hand by a single Scion and the blades
are crossed, either nothing happens, or that person is shifted to a random
plane (DM choice). Also, the wielders might wish to be careful not to duel,
or practice against each other while wielding these swords; I'd recomend
using different weapons on such occasions. One never knows. . .

Any suggestions folks may have to tweek the powers would be welcomed. I
based them, as I said, on swords found in Harry Turtledove's Videssos
Cycle, and also on the powers of the Holy Avenger from the DMG. The swords
are not intelligent, per se, but they should be considered NG in alignment
(where such things matter).