Burn is an effect applied to enemies that sets them on fire and then deals damage to them over time. Only certain units have the ability to apply burn.
When one of these units hits an enemy, the enemy takes damage from the unit's normal hit damage, and adds the burning effect on the enemy. Enemies can only be under the burning effect once, so any additional hits by one of these units will only cause the normal hit damage. Once the burning effect ceases, it can be applied again on the following hit.
When an enemy has the burning effect applied, every two seconds it takes damage from the burning equal to the initial damage hit that applied the burning. Burning lasts for 12 seconds, and thus applies an extra 6 ticks of burning damage over time.
To maximize the damage of using multiple burning units, it is best to stagger them along the path at the correct intervals for each new unit to inflict burning damage again as the enemy goes past. If the unit's attack style is 'single' or AoE (Circle), their priority is best set to strongest in order to attempt to apply burn to as many enemies as possible. For AoE (Cone), it is advised to set the priority to 'Last' or 'First' as applicable. Burn can stack with bleed, black flame and poison damage.
List of burning units
About Black Flames
Is a special case, Kosuke (Eternal) can apply “Black Flames” with his special ability, although this effect is different from burn and bleed and can stack on top of them. When activated, the Black Flames effect is applied to all enemies within Kosuke's range, and deals 9% of his damage to each unit per 2 seconds. This effect lasts for 40 seconds.