Nourishment(2 m): +10% damage vs undead
-10% damage from undead
+10 Experience from kills
The duration is supposed to read (2 h). When consuming a potion the duration of the buff has also been reduced. They last for one hour, pretty sure last patch they where lasting for 2 hours.
There could be a problem with all the extended potions of slaying, but I only checked this one.