@ReptartheGrand first, welcome to the boards.
Second, generally anything people ask if it is too powerful- it is.
I never heard of a 5e item that requires you to have a 18 stat to use. It might be a fine restriction though. It seems like it is just for rogues to be able to get a second attack. The first deals backstab damage and then you get advantage to deal 1d8+4 more? Does not seem terrible. I would likely simplify it to allow you to make an extra attack 1/rest or 1/day instead of each round.
Or, just have a weapon deal 1d8 fire or whatever. Or, just skip the extra bits and just have a rapier that lets you make an extra attack each round and hope the fighters do not take it from you. Or, just use a scimitar of speed. I'm not a big fan of dex weapons dealing 1d8 anyways.