In the game I was running last night the bad guy cast invisibility on herself. The players managed to work out which 5' square she was in and among other things one of the players tried to use the spell Command to command her to approach.
I made an on-the-spot ruling to allow it (she made the save anyway) but I couldn't find anything in the rules to say definitely whether:
- The spell works as usual
- The spell works but has a 50% miss chance as per an attack
- The spell fails
If it was an AoE spell that would be simple (the spell works).
If it was a ranged touch or touch spell that would be simple (50% miss chance).
But the range is close and the target is one living creature. Does not being able to see the creature despite knowing roughly where it is prevent spell casting on them entirely?