A Spark in Shadow is a Warlock quest that unlocks Stormcaller subclass.
"The Vex are practical, focused. They store energy in all its forms. The Conductive Mind contains a powerful Arc source. Seek out this construct, destroy it, and claim the source as your own." - Ikora
Head to the tunnels under Freehold on Mars and face down the Conductive Mind for your own source of Arc. Complete mission "A Spark in Shadow".
