do I get to keep this thing ?

also how do I join the companions I was at the place talking to soem guy then told me to follow him into the backyard area then he just turns around and starts attacking me so i retliated with magic(cuz im a Mage) and he isnt taking any damage ... so I ran away and havent looked back

Yes, its for the restoring the Gildergreen tree quest you get from the priestess of Kynareth in Whiterun.

You can keep Nettlebane after you do the rest of the Gildergreen quest.

If you listened to the first old guy you talked to, Kodlak, he tells the other Companion to take you outside to test you in combat. Since this is a sparring match, basically, he takes no damage. He actually tells you to hit him. He shouldn't start attacking you.

Being a mage might not be the best thing to be if you want to join the Companions.

Yeah, I can see missing Kodlak's advice, but Farkas will stop and tell you "what's this magic stuff?!" and demand you use a weapon like a proper fighter. Yeah, the Companions are not at all into magic, and if your style is assassin they'll criticize that, as well.

this is why whenever i see them i sniper them down to their knee's that and there dirty dogs in disguise, being a vampire is alot cleaner after all...

also if you see them out in the wild helping a settlement which i have done a few times if you dont help them they will call you a coward or something along those lines.