The one in Forgotten Realms Campaign Setting is a bit better, though. It goes up to +15 damage, +30 to undead. Hathran also get it at level 3 slot.
Still, concentration DC 10+Damage Dealt+Spell Level basically means that the target cannot cast, if your damage is anywhere optimized. It should also stack, so if you get multiple hits, you force multiple checks. And since it's any kind of sword, you can dualwield them. At CL 5, a "lightsaber", I.E: At-will item of Moon Blade, would cost 5x3x2000. 30,000, and it's a fairly decent weapon to begin with. Assuming, of course, that your DM allows it. You could reduce that to 18,000 by saying it was made by an artificer and is only CL 3, but meh. Either way, that thing can create two of the blades, so you'd get a two-ended lightsaber for fluff, if you wanted to run dualwielding.