Stockvillain
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Pathfinder Adventure, Rulebook Subscriber
All kinda depends on what you want it to accomplish, really. Consider exiting spells and compare it to them. You want raw damage? Fireball seems to be a good starting point. 1d6/level negative energy sounds on par with a 3rd level spell. Add in a couple of status effects to distinguish it [Sickened/Nauseated with Fort saves], shorten the range to Medium [because Long range is crazy], and you should be fine. Again, tweak for flavor as desired.