if you slow someone with nasus wither and red buff

the higher slow would take dominance.

It was changed recently so that slows do not stack; only the strongest applies (Wither, in this case), although when it expires, if the red buff is still active, it'll take over.

Ever since a certain patch, they changed how slows were, so the strongest slow will be the slow that takes effect. This was also the patch got buffed.