The key is in the sewer in the first level of the Baldurs Gate Dark Alliance.
Chat with our AI personalities
Pass by save point
You first get a weapon on the level called Jailbreak. The first weapon is a crowbar, then later in the same level there is a pistol and shotgun.
at the mine camp you must unlock it
I'm afraid it's impossible to get the onyx sword in Dark Alliance II, but it can be obtained through item replacement using a PlayStation 2 gameshark. I don't know the code though.
You go to the onix tower and on the third level i think before you fight eldrith you fight this big mechanical guard of some sort and when you kill him you should get the onix sword