Flamestrike

Flamestrike is a GTAoE spell that deals damage on initial casting. After the initial spellcast, damage will tick on the targets that remain in the area.

Talent improvement
World in Flames increases the chance that Flamestrike will critically strike by 2% per rank, up to 6%.

Firestarter allows an instant cast Flamestrike every time a damaging Blast Wave or Dragon's Breath is cast.

Flamestrike deals fire damage, and thus gains the benefit of the talents Impact, Ignite, Burning Soul, Master of Elements, Critical Mass, Fire Power, Pyromaniac, and Precision.

As with all mage spells capable of critical hits Arcane Instability, Spell Power, Molten Fury, Playing with Fire, and Shatter can increase the effective damage output of this spell.

Tips
Outside a few specific situations this spell is fairly useless. The area it affects is far smaller than that of Blizzard and Arcane Explosion and a small fraction of that affected by the warlock's Seed of Corruption. It's difficult to have a situation where targets stay in the affected area, since they usually get mad at you and run out of the area to let you know how they feel about massive fire damage. To get the full DoT effect (which is almost half of the spell's total damage) you'll need to cast this spell and then run into the pack of things you just angered. Mages with Arcane Impact and Blast Wave are very very nasty up close, but in a PvE raiding context it is unlikely that the mage will survive such a tactic. An alternative is to snare or root your targets somehow. You might think that Frost Nova would help here, but since its rooting effect has a chance to break when the target takes damage, it isn't perfect, and because of the small radius of the Flamestrike it hardly ever affects all mobs which have been targeted.

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.