The main effect of volcanic eruptions is

The main and most immediate effect of a volcanic eruption is the flinging of ash and other ejected material into the air.

Mud slides are a possible secondary effect but not the primary one described here.

Lower surface temperatures can follow major eruptions through ash veiling, yet this is an indirect after-effect, not the main one.

Since the flying ash is the primary effect, all of the above overstates it.

Heavy ash falls damage crops, choke water bodies and disrupt air travel.

Ash and gases ejected high into the atmosphere can spread over vast distances.