How to Cast Drain Soul

[viva111 0]

Hi,

i created my own fight class for warlock but i cant figure out how to cast drain soul. 

i have the following conditions for it: 

- Soul shards smaller 3

- Target health percent smaller 25

After i casted my other spells correctly i just cast shadow bolt till the mob is dead.. and i need something like interrupt spell cast for drain soul..

Any help pls?

Oh and does anyone know how my voidwalker could cast consume shadows if life is under __ when out of combat till life is up to __ ?