It seems like there's something missing from this spell, as it states "Will partial" as the save but doesn't go on to explain what those partial effects would be. Has this been addressed anywhere? I've had a hunt around but couldn't find anything.
https://www.aonprd.com/SpellDisplay.aspx?ItemName=Betraying%20Sting
My other reason for suspecting there's something missing is that it's extremely underwhelming: 1d8 damage per 2 levels, against a single creature, who is indifferent or better. While the long range is nice, this spell also seems to require that you spend time secretly ingratiating yourself with an enemy before you can cast it. There aren't that many situations I can think of where Medium to Long is a major advantage (close to medium is a different matter), and you've still got to have LOS to target them.
OK, I get that it's probably intended for treacherous NPCs... but it'd be nice to know how it's supposed to work and what the saving throw is for. It feels like there should be some significant betrayal-themed kicker that goes along with a failed save.