I think the raid has a minimum light level of 280 but that doesn't take too long to reach and you can get checkpoints at certain parts of the raid. Nightfalls are just harder strikes that have different modifiers each week and a time limit. They give out higher light weapons and armor through luminous engrams, you only get the rewards the first time you do it for the week unless you've got more than one character. There's a prestige mode Nightfall that makes it a bit harder to complete but its not worth doing. You're also going to want to do them with 2 other people since they're basically impossible to solo in D2. As for getting a better sword its just like getting a better weapon or piece of armor, it'll drop randomly. I'd recommend doing the public events on Nessus since they're the most popular at the moment, you should look up how to activate the heroic versions of the public events too, you'll get more rewards and a chance at an exotic engram.