How much Damage would a Lightsaber do in D and D?

[FMArthur]

If what you have in D&D after it's ported over don't match what a lightsabre should be able to do, then you don't have a lightsabre. You have a glowing longsword. OP asked for a lightsabre - that's about what it needs to do (translated over to D&D rules) to be considered one. As for their danger, you can hold a blade of a sword in your hand, of place the blade against your body given certain techniques. Try that with a lightsabre, and it'll end poorly for you - thus the requirement for a fumble rule. Lightsabres are explicitly fluffed as being potentially dangerous to their wielders, especially if they aren't highly-trained force-users.

If you just want a glowing longsword, then put a +1 enchantment on it and have have it glow.

Okay, so it's an exotic longsword that takes extra practice. Double bladed swords and spiked chains don't have a chance to harm nonproficient wielders, though, so why should this? If what you want isn't reasonable in the system you're porting it to, why bother? Do you just want an overpowered weapon that disintegrates on contact?