Find the nautiloid control room during the prologue
Interact with the pod containing Shadowheart
Use the nearby console to release her
If the console requires a key, loot the key from the nearby cambion or open the adjacent pod controls
After freeing her, speak to Shadowheart
Recruit her to your party before leaving the nautiloid
If you miss her on the ship, find her at the Druid Grove area later and recruit her there