The Devil's Sight Invocation for warlocks isn't working properly. I'm currently in combat in the overgrown ruins area, and my warlock has both advantage AND disadvantage against targets while he's in his magically-created darkness. He should have advantage on attacks against creatures in the darkness area and they should have disadvantage (because they are blinded and he isn't). I'm playing a Seldarine Drow if that makes a difference (since they have Superior Darkvision, which might override Devil's Sight in its entirety and not just the darkvision portion of the Invocation).
