After Jack made a fittingly morbid return to Winchesters' mother and forcing people to always tell the truth. Thankfully, Jack was talked around just in time for a showdown with God in Supernatural's season 14 finale - a confrontation in which Jack was immediately destroyed by his grandfather. The nephilim's survival was teased in a final scene that showed Jack waking up in The Empty with Billie, the current guise of Death, who told the boy he still had work to do.
The scene heavily implied that Billie had recruited Jack to help with the oncoming threat Chuck posed to the universe, but the powerful duo remained absent for the first block of Supernatural's final season. Jack and Billie reappeared several episodes ago, but it wasn't until this week's "The Gamblers" that Jack properly returned to Earth. Launching a comeback in somewhat non-traditional fashion, Castiel first discovers Jack journeying around the United States and munching angel hearts. Initially hesitant about Jack's revival (understandably so, given the Belphegor debacle), Castiel soon discovers Jack's intentions and learns that his return is genuine.
First, the audience discovers why Jack hasn't been around in Supernatural season 15, despite Billie bringing him back to life all the way back in season 14's finale. Jack's return must be kept a secret from God, otherwise Chuck will just swoop in and re-smite his grandson, so the nephilim had to wait until God was in a different world before making his move. Straightforward enough, except God couldn't traverse universes while he and Sam were linked by their Equalizer bullet wounds. That issue was only fixed 2 episodes ago, meaning Jack's return was impossible until recently; even now, the honorary Winchester can't use his powers for fear of being sensed.
This explains the timing of Jack's return, but what's the meaning behind it? According to Billie, Jack's status as a nephilim means he harbors the potential strength to take down God, but he'll need a considerable power-up to reach that level. This is why Jack is found eating the angel hearts - presumably to inherit the grace of his victims. Despite video evidence suggesting otherwise, Jack isn't simply slaughtering angels at random, of course - Billie is directing him towards the Grigori. Introduced in Supernatural season 10, the Grigori are a rare group of angels living on Earth, some of whom feast on the souls of the living. Jack takes down one Grigori posing as a doctor and consuming the souls of patients, and another who prefers the taste of the young. By hunting a monster and powering himself up in the process, Jack is killing two angelic birds with one stone.
If Billie's plan is to pump up Jack until he's strong enough to fight Chuck, this could tie into the theory that Jack will actually become the next God. Supernatural has already confirmed that God can't be killed without endangering the universe he created, and his absence would result in a sharp rise in the monster population, as highlighted by Sam's flashforward. If Billie could take over as Death, perhaps Jack can do the same with the role of God, and therefore Chuck can be defeated without upsetting the natural balance of Supernatural's world.
Supernatural continues with "Galaxy Brain" March 17th on The CW.