How to make Shock Wave etc chance cast

[AznShadowFury]

Hi, I want to know how you can make certain spells chance casted, e.g. in DotA the maelstrom Item gives a chance to cast chain lightning on each hit, how do I do that? Is it by a trigger or something?

 

[donut3.5]

Trigger or Orb ability.
With the orb ability, you can base it off of orb of slow, and then... well, its pretty self explanatory from there.
--donut3.5--