The AI will use all custom spells like it uses normal spells (they inherit the same AI characteristics). Thus spells based on shockwave will automatically be cast in combat. Spells based on death pact will never be cast (no AI for these spells).
You can always order units to cast them via triggers. Be aware that GUI is retarded and is confusing when it comes to doing this. Triggers actually issue orders to the unit to cast the spell via an order string or an order identifier. In GUI it looks like you can only order non custom spells but actually it is refering to the order that non custom spell uses. Thus ordering a unit to use tauren chieftain shockwave in GUI will also order it to use a custom shockwave.
If you use JASS, you will realise they share the same order string both abilities (custom and normal) so this is less confusing. Although you can change the order string for most abilities, it will mostly never work and almost always still use the order the parent had (channel being one of the few exceptions).
