If you click on the spell description it says "Increase Aggro Modifier by 10%" but up there under the spell name it says "Decrease Agro Multiplier by 90%" and I checked Lucy and it says the same thing, so it's probably miscoded right now.
Til it's fixed, that's a great way to keep Shammie's and chanters alive while they're slowing mobs. The enchanter line of aggro reducing spells aren't that high a percentage and they only last a few tics, this lasts for 10 minutes.