In most of the versions of the spell it was quite powerful, needing a caster of skill to be cast.
The Oldest known version of the spell had the caster chose between the effects "Charm", "Fear", "Sicken" or "Sleep" when casting. When looking at a creature and speaking the command word it would effect the target.
Charm would attempt to make the target obedient and loyal to the caster, Fear caused terror in the victim making it attempt to escape. Sicken would attempt to weaken the to half of its ability and it would move half as fast and Sleep would attempt to cause the target to sleep.
A later version would have assaulted a target, causing more ill effects on weaker targets and less so to stronger once. "Sickened" would have effect any one whom the spell succeeded on and it caused the target to be less able in combat and with its abilities or skills. "Panicked" would have been applied to the moderate or weak, It caused the target to panic and would become shaken for at least 10 minutes and if it saw the caster during that time it would panic again. "Comatose" caused the target to sleep for atleast 10 minutes, as this was not a sleep effect it could have worked on elfs.
After the Sundering the spell changed again this time the casters eyes would have changed to a black void. If the caster could see a target within 60 feet, it could use its dread power on it, choosing between "Asleep", "Panicked" or "Sickened". "Asleep" simply attempt to make the target fall sleep, "Panicked" attempted to make the target always try to get as far away as possible from the caster as possible. "Sickened" tried to make the target disadvantageous in combat and with its abilities.