Effect
Pierce the Veil
Pierce the Veil is a passive that noticeably raises all of your damage dealt — but at the cost of significantly higher Mana costs. It's a classic risk-reward trade: more punch in exchange for tighter resource management.
The slot pays off mainly when you've got your Mana problem under control elsewhere, via Mana regeneration, health-globe effects like Gruesome Feast, or cheap primary spells as your main damage. If you instead rely on expensive, Mana-hungry skills, the surcharge can seriously disrupt your flow. In summoner builds whose pets deal damage independently of Mana, the passive is especially attractive, since you take the damage bonus without feeling the higher costs much.
The math behind the surcharge
The passive trades 20% more damage on everything for 30% higher Mana costs. What matters is which of your skills cost Mana at all: Spirit Walk, Soul Harvest and Horrify run purely on cooldowns and are unaffected by the surcharge. Piranhas and Locust Swarm, on the other hand, do cost Mana and get pricier accordingly. The more of your bar consists of free cooldown skills, the better the trade.
Pierce the Veil unlocks at level 27. While leveling, with a small Mana pool, you feel the extra cost more sharply than later with developed gear.
Mana support in the package
Grave Injustice is the most common passive companion: every enemy dying near you returns Mana and life and shortens all cooldowns. In dense packs this often offsets the surcharge on its own. If the numbers stop adding up in long single-target fights because hardly anything dies, that is a clear sign to swap the passive for a more economical option.
How to tell it fits
Watch your Mana orb during a typical fight: if it stays above half while you keep casting Locust Swarm and Piranhas, you are getting the damage bonus practically for free. If it regularly drains to the bottom, the passive costs you more damage than it brings in.