Moonblade is given by the Royal Huntsman in act 5 for bringing in all 10 trophies throughout the game . NOTE : after turning in all 10 trophies and initiating a second conversation you are given the choice of 1 of 2 swords
G`valchir or Moonblade

Its Dagon, the evil God for the rebels vodyanoi.

Um. I was looking the steel sword list http://witcher.wikia.com/wiki/Steel_swords_in_The_Witcher , and I have some doubt choosing the best. Why do you say G'valchir is better than Gwalhir? Though G'valchir does +100% dmg and causes armor pen, Gwalhir has some chance effects (disarm, pain and precise hit).

Finally, meteorite swords (or even rune swords) are really useful? Steel swords cannot be upgraded, and the meteorite sword stats are commonly worse (with the exception of the witcher's steel blade). So, in my case, i'm in Chapter II, and I have Harvall steel sword. Checking the ores, I think I can't get a meteorite sword better than Harvall, am I right?

And about silver swords, if I've read correctly, forging a rune sword http://witcher.wikia.com/wiki/Rune_swords (I play the Enhanced Edition) will REPLACE the base stats of the sword used. So, are they only useful until you get Aerondight?

Well, thank you for reading!

Steel swords: I prefer the critical effect as well. I typically forge a 3 color-meteorite sword until I get Harvall (good job getting it so early, those Wyverns are tough). Then I use Harvall until I get Gwalhir. But I also use FlashMod which nerfs G'valchir's damage bonus, so I usually don't even consider it.

Silver swords: there is no "base sword stats" with steel or silver forged swords - the swords are forged solely from the runes or meteorite ore. Typically I only have time to forge a single rune sword (I choose 3 color runes) before you get Aerondight. Then there's nothing that surpasses Aerondight.