Effect
Your damage dealt is reduced by 20%.
Heavenly Strength
Heavenly Strength is a passive that enables an unusual weapon combination: you may wield a two-handed weapon in your weapon hand while still carrying a shield. The price is a sizable cut to the damage you deal.
This passive is the foundation for a whole build family, because many strong Crusader set bonuses and legendaries rely on the high base damage of a two-hander without forcing you to give up a shield's block and defense. Despite the damage penalty, the upside from higher weapon damage and extra shield defense usually wins out clearly. You take this passive almost any time your build is built around a two-hander plus shield; in pure one-hand setups it offers nothing. It isn't a damage booster, then, but an enabler for a specific gear concept.
What the 20% penalty means
The passive lowers all the damage you deal by 20%, no matter which skill lands the hit. It only pays off when the two-hander and shield together bring more than you lose, for instance because a legendary power on the two-hander is essential to your setup, or because the shield itself carries a power you don't want to miss. If neither applies, another passive slot is usually more rewarding.
Since the skill is available from level 10, you can already test the concept while leveling, as soon as you find a usable two-handed weapon.
Making use of the shield
If you carry a shield anyway, your skills should benefit from it. Iron Skin is an obvious partner: with the Crusader shield Hallowed Bulwark it also raises your Block Amount. Laws of Hope places a damage shield on you and your group, and Long Arm of the Law extends its active phase.
Offensive partners
Many players offset the penalty with amplifiers: Akarat's Champion raises your damage and Wrath regeneration for a while, and Finery grants Strength for every socketed gem. Steed Charge provides the movement you need between packs.