Vaal Pact and Power Charge

Is Vaal Pact worth on Spellcaster(did I write it good?) Shadow ? Ice Nova +two daggers(crit build)

What mean power charge ? when is it active ?

Vaal Pact is worth it when your life leech from damage is very high compared to your life/ES pool.

Power charges give you extra crit chance. You need specific items or skills to generate them. You have a power charge counter icon on your buff bar, and blue balls floating around your character when they are active.