Haste and Slow are opposites of each other.
Our group got attacked by creatures under the same haste spell. Our caster responded by casting slow on them but due to their spreading out was unable to hit all the ones under the effects of haste, and also hit some that were not under the effect of haste.
Slow
Slow counters and dispels haste.
Haste
Haste dispels and counters slow.
We were unclear on exactly what happens. We came up with the following potential cases and were not sure which was correct
- The whole haste spell is dispelled and no other effect
- Haste is dispelled and others can be affected by slow
- Only the ones within the slow area are dispelled of haste or slowed
There is a related question which was dispel magic and multiple summons from the same spell.
Best Answer
Here the Pathfinder Core Rulebook FAQ directly addresses this issue:
Thus, in your case, after picking the targets of the slow spell, those folks who were affected by the haste spell automatically wouldn't be affected by the haste spell any longer and also wouldn't be affected by the slow spell, but those folks who weren't affected by the haste spell would make saving throws against the slow spell normally.