Playing Shadow Demon late game

Hi, I was wondering how it is that you guys play shadow demon late game. Usually I can own with shadow demon early on and rack up the kills, but I cant win a game with him because I become useless in team fights and in the late game. I dont go shadow poison first. Usually i go fast urn, amplify damage, and disruption. I got this strategy from a game purge played with shadow demon on youtube. Suggestions anyone?