I've been running a campaign in Mystara for the last 13 years (which is currently on-going but the sessions have become very rare) and I've sketched a lot of magical items, especially weapons. Just below you'll find one of them (2nd edition rules):
Lughdegar
by Marco FossatiDescription
Claymore sword (just like a two-handed bastard sword) The blade is more than 5 feet long and made by fine grey steel. Five yellow precious stones are laid on the top of the hilt which is black. Its scabbard is also black with a yellow sun with sunrays carved on it.
History
Some sages think this sword was created on Laterre while others this it was on Mystara in the first years McGregor's came from their old home-world. Everyone however agree David McGregor crafted and gave it to William McGregor, second son of his half-brother Robert. Legends say William (who had no magical skills) was gifted with Lughdegar cause his uncle wanted to help him since he was one of the officers of Lord Glantri during the war against Halzunthram. with Lughdegar's powers (see below) William McGregor performed a couple of great tasks like surviving to an ambush at night against 1 Alphatian mage and 10 soldiers and leading a charge against a company of bowmen in the final battle between Glantri and Halzunthram.
Since then the sword belonged to McGregor's clan passing between clanmembers without magical abilities. Soon, while the magocracy stayed away from wars, the memory of Lughdegar began to fade but it remained strong among the members the Followers of Claymore.
It reappeared during the recent war against Ethengar in the battle of Skullhorn Pass in the hands of Duncan McGregor. While fighting against an Ethengarian mage in a personal duel, the spell cast by the hakomon (some told it was a magic missile spells with 6 missiles) simply took no effect and Duncan easily slew him. Again the powers of Lughdegar helped one of the McGregor's.
Powers
It has +3 bonus to hit and damage.
The wielder of Lughdegar has the permanent effect of the wizard spell Shield and he's immune to normal missiles.
Twice a day it can cast by will Light (as 1-st level spell) and once a day by will Infravision.
Some old tales say by yelling the name of David McGregor some powerful offensive spells are cast but there aren't proofs about.
It's almost certain, but yet not confirmed, Lughdegar can be wielded only by a McGregor.
Campaign Use
IMC members of the Followers of the Claymore revered Lughdegar as a relic. The wielder can use it to increasing the number of the Followers.