The setting is a traditional fantasy setting: magic, elves, medieval level technology/culture, etc. The protagonists are siblings: an older sister and a younger brother, teens to young adults. The brother discovers he has an incredibly powerful raw talent for magic. The drow (is that term trademarked by Dungeons & Dragons?) or dark elves have made many offensives in the area where the characters live and taken many lives and land. The brother is unsure what to do and turns to his older sister who is a veteran of a previous battle with the elves (ok, so she’d have to be a few years older). He is a bit naive, trusting, innocent. She is hardened, cynical, tough. She brings him into the woods to a secretive teacher of the arcane arts. While something about the mentor doesn’t sit right with the boy he decides to trust him. The way the teacher harnesses power seems unsubtle, crude, forceful, but powerful. In short order the boy begins to harness great power.
The back story is that the teacher is a dark elf and is working with the sister. They are promising her great power when they take over the area, and safety for her and her family. She is so tired of the battle and bloodshed she believes they’ll keep their promises. They say only they need her brother as a weapon and he’ll be unharmed, and in addition he’ll learn great skill over his power.
Their plan is to trick the boy into thinking he can cast a great spell to defend his town. In reality they teach him how to summon great energy but not release it. Thus, he becomes a sort of ticking time magic bomb once he’s summoned the energy (to defend his town from a faux attack). The elves then warn the villagers they will only help the boy release the energy harmlessly if they lay down their weapons. The boy realizes the betrayal, and the sister tries to explain.
Once this is done, they imprison all the villagers and stop the boy’s power by killing him. The boy in his last breath forgives the sister. Realizing her own betrayal at the hands of the elves, she rushes forward to comfort his last moments. Unbeknowst to her she also has great magical power and heals him. Together they turn back the drow and protect their town.
The basic message is one about power and trust, betrayal and love.