You can also capture this skill from a white mantle female boss called Manton the Indulgent in the very same Aurora Glade mission.
I noticed that the bosses in this mission as in many other missions will be found in different places. Therefore u might or might not find Cuthbert the Malignant (or similar) right before crystal part as Hazar noted.
You can buy this skill from skill trainer Gammel in Copperhammer Mines (Shiverpeak - higher levels).
It does marginally more damage (30-40 or so) over time than Rodgort's Invocation, but if you only had one slot to choose between the 2, Rodgort's should usually be chosen, for its shorter recharge and up front effect.
Particularly now that mobs won't stand in AoE effects like Searing Heat for long.
Now the aoe has been modded id like to see a buff to this skill. Somehow though its too similiar to firestorm. Personally, i think that it should just set the aoe on fire for x seconds. 14dps armour ignoring vs 35dps before armour.... that would be a bit more like it... still wouldnt use either though.