Power increases a hero's auto attack and ability damage by a percent. A hero start at 100% and that increases as they get power.
This along with attack speed and attack damage are the only stats that can increase damage.
If a player has 40 damage and 150 power, their actual damage is (Damage * Power / 100), so one deals 60 damage.
- Bracer of Strength
- Crystalline Shiv
- Diadem of Serenity
- Fel Blade
- Icon of Destruction
- Rune Boots