It is just an illusion, a figment of light and imagination. But it is so thoroughly convincing that people have been known to smell it's illusionary sweat.
Phantom Image is a fourth-level Order Magic spell in Heroes of Might and Magic IV that costs eight spell points and creates a duplicate of a single creature. The strength of the image depends on the caster's level.
This spell harbors twice the power of Create Illusion for slightly more than twice that spell's cost. Thus in some situations you might want to use Phantom Image's weaker cousin. For instance if you're trying to get absolutely as many illusions for your spell points as possible and aren't in any danger of being swamped by enemy creatures (eg. in siege combat where the enemy is not moving out, or where you have all opposing stacks under control with Slow and Teleport) then you might want to cast the less powerful illusion spell.
When cast on a stack already summoned by Phantom Image or Create Illusion, or a stack of the same creature type, it adds to the stack already summoned.
In the Equilibris mod, Phantom Image is a third-level spell.