Illidan's Footsteps

How can I have these footsteps? Is there a spell or sth like that? Because I want to add it my Hero while hes casting Bladestorm. He shall have these steps but in form of flames, like a fire walk. Enemy units who get in contact with the flames shall have reduced armor and take damage over time. I'm very glad if someone can help me

You can create a trigger to spawn a unit with the footstep model at the position of the caster every few seconds as he's moving. Give the unit a low-radius negative aura of whatever you want it to do to your enemies, and give it an expiration timer so it fades away.

The reason Illidan's TFT model has these footsteps is that they are an event defined in the model for whenever the feet collide with the ground. It's not an ability.