Flamestrike is a core mage ability learned at level 44. It is a ground-targeted area of effect spell that deals damage on initial casting. After the initial spellcast, damage will tick on the targets that remain in the area. Note that mages with the fire specialization also debuff enemies in the area with a slowing effect.
The DoT effect only damages those who stay within the area. The ground gives off a "pulse" of additional fire damage that burns all standing above the effect. Those that wander out suffer no additional damage, and those that you missed on the initial casting take damage if they later wander into the spell's area of effect. The borders of the effect are very clear; the ground is laced with veins of red hot energy.
It is also notable that Flamestrike's damage is not reduced because of damage to the caster like [Blizzard] (because Flamestrike is not channeled).
The 5 pc. set bonus from the Netherwind Regalia set will increase the radius of the ground-target area by 25%, for a 6.25 yd radius.
Tips
Flamestrike can be useful for mages serving a DPS role for groups in instances. Once your party's Tank has sufficiently gained aggro you can use Flamestrike with relative certainty of hitting your targets for optimal or near optimal damage. This is advisable only in parties that are familiar with each other, as it can lead to the death of your mage and possibly a wipe if aggro on the tank is shifted to the mage.
Patch changes
Patch 5.3.0 (20-May-2013): Flamestrike now deals 100% more initial damage. Periodic damage remains unchanged.
Patch 3.3.3 (2010-03-23): Some ranks of this spell had an incorrect cast time of 3 seconds. All ranks now share a 2-second cast time.