We moved into our house 3 months ago and before that the owner was out of state and the house sat empty for 6 months. Not sure if that has anything to do with it but there is a horrible smell coming from one of the floor drains in the Unfinished basement. I have poured gallons of water down it a couple of times and it seems to make it worse. It is not dry, I can see the water sitting in the trap. I'm also not sure if it's related but I found drain flies in the bathroom located beside this drain. I am in the process of figuring out where they are coming from. Any suggestions and help is greatly appreciated.

pour peppermint oil down the main stack. then some hot water. the person who did the pouring should not go into the house. now, smell around to see if the peppermint smell is coming thru from under the toilet or from the floor drain.

Ok and what does that mean if it comes to one of those?

Pour a quart of water into EVERY single drain. Floor drains should have cleanout plugs installed in the side of them; if they don't, that is where your sewer gas is coming from.

when a homeowner complains of a bad sewer smell in a house and it is difficult to pinpoint the place it is coming from, then some kind of testing has to be done.
easiest test is peppermint oil and a good strong sense of smell.
next type test is a smoke test.
this will give you an idea of what that is...
[www.bing.com]
smells in a house can be very difficult to find..

I have done that several times.

I know exactly which drain it is coming from. Now I need to get rid of the smell.

as said, some floor drains have an accessable clean out under the cover. make sure there is a plug screwed into the cleanout out opening.