Command gives your Hero effects on his troops around him, as well as deciding the distance at which these effects can be inflicted across and their duration. It also decides the distance at which your Hero can convert buildings and the range of his spells.
Every Hero has a Command Radius that can be displayed any time by pressing the R key on the keyboard. The Command Radius is the single most important element to a Hero's Command - when ever your Hero plans on casting spells or converting buildings, only targets within the Command Radius (or touching the edge of the ring) can be targeted but only up to 32 targets.
Effects on TroopsEdit
When unit's from your side get within your Hero's Command Radius, they'll receive +4 combat and +2 speed. When under these affects, they'll also display a shining star icon above them. However, once the unit leaves your Hero's Command Radius, the effects will wear off over time and the duration is actually dependent on your army's Morale (1 second for each point of Morale your army has).
For every two points invested in your Hero's charisma stat, his/her Command will be increased by one point. Once you reach 12+ Command, your Hero's Command Radius will be at its peak and can no longer be increased in size, making further investment redundant.
The Undead race have access to a research known as Dark Lord that increases the Command of their Hero by +8.