Freed souls appear during the Nomad's Elegy when fighting Gielinor as Death and Icthlarin. When playing as Death, he must use Provoke on Gielinor to cause it to slam its fist into the ground near Death. Death can then use Scythe Swipe on Gielinor's arm to tear a freed soul out, which he must then claim with Soul Reap.
Freed souls are again encountered when playing as Icthlarin. They are found in clusters of seven on the floor around Gielinor, and must be claimed using Icthlarin's Release Spirits ability before Gielinor claims them with his beam attack. Icthlarin requires more spirits than Gielinor in order to survive and reflect back Gielinor's final attack with his Shield ability. Depending on how many spirits Icthlarin claims, he may have to repeat this process. Claiming all 49 spirits at once allows Icthlarin to reflect all of the damage and end things in one cycle.
This information has been compiled as part of the update history project. Some updates may not be included - see here for how to help out!