Research Banner Carrier is a unit-upgrade common to all nine factions in all three BFME games. Banner Carriers increase a unit's level immediately to Level 2 - which brings a small increase to the unit's damage and health-points.
The cost of this upgrade is always 1000 resources.
The Banner Carrier himself stands before the front line of the battalion, and any battalions at Level 2 or above who have lost members will gradually grow in number back to their original warrior-amount. (This does not mean that units heal - only Elven warriors heal over time.)