Mid. Use the observer ward you start with to ward the enemy mid's hill (keep the ward out of tower range) so you can spam Enfeeble on them. Enfeeble pretty much prevents the enemy mid from laning at all by reducing their damage to nothing. Every single time Fiend's Grip is ready, find an enemy hero to use it on, then hold R to channel it. It's a guaranteed kill if their teammates don't disable you. NEVER tp to a lane if you aren't planning to kill someone there, you are insane in fights and need to keep your TP ready for that. Nightmare is a long disable but only you can damage nightmared heroes without waking them (your allies can't)