Single Target Slow Ability w/ the slow amount changable?

I am looking for a way to make a unit that attacks and causes a unit to slow down, then when the level goes up the slow is even slower. I looked at frost attack but that doesn't seem to let you set how slow it will make the target I also saw one or two attack with the slow buff, but they didn't let you set the speed, nor does the "slow" buff. Is there an ability that I am missing or would this have to be done with triggers? (I know unholy aura can set the slow amount but I want it on an attack not an aura).

edit: oh yeah, it can't be poison, cause I already have a normal poison attack too. And it needs to be passive.

Data - Attack Speed Factor
Data - Movement Speed Factor

Change those on the spell to change the effect.

Ah, I think slow orb may do it, haven't tested it, but it looks like it will work. Thanks.

you could use slow poison eh?
i mean for normal poison attack you use envenomed spears right?

anyways the frost slow factor is in gameplay constants... you can only have 1 value for it..

just wanted you to know.. but i guess you already found out.