Remains in play. This spell has a range of 24" and can be cast on an enemy unit visible to the caster which has no models (friend or foe) within 1" of its front rank (walls. hedges and other scenic features don't matter). The spell cannot be cast on units with a 360° line of sight.
A searing wall of flame suddenly rises in front of the unit. To represent this take some cotton wool or paper and place this in a line up to 1" thick in front of the unit.
The unit suffers 1 automatic hit for each model (including characters) in its front rank. Each hit is resolved with a Strength of 4.
Once it is cast, the Wall of Fire remains where it is until it is dispelled, or until the Wizard chooses to end it (which he can do at any time), attempts to cast another spell or is slain. No further hits are caused by the Wall of Fire unless a unit tries to move through it, in which case each model that moves through the Wall of Fire suffers a further Strength 4 hit.
The Wall of Fire does not block the line of sight of archers and chargers, nor does it reduce a unit's Movement rate.