Her Tamed Lightning is an ability that creates a shield of elemental electricity around the target's body which increases resistance to Shock Damage, and inflicts a small amount of damage upon nearby adversaries.
Her Tamed Lightning is offered as a reward for The Missing Hand if the Nerevarine kills Salas Valor prior to obtaining the quest. However, due to a bug, Her Guard Against Terror is received instead. There is no alternate method of obtaining this ability during regular gameplay. It can only be acquired on the PC by using the console:
Player -> AddSpell "her_tamed_lightning"