Thanos once faced off against his son Thane and lost everything as his secret child shockingly overpowered Marvel's Mad Titan.
Warning! Spoilers for Thanos (2015) below!
Thanos' secret son once gave his father a taste of his own medicine, unleashing a powerful blast that left the Mad Titan powerless and made him a mere mortal. In Thanos (2016), the iconic villain came face-to-face with his son Thane, who responds to his dad by using the Phoenix Force (with Death on his side) to completely humiliate his evil dad.
In the excellent Thanos miniseries by Jeff Lemire, Mike Deodato Jr., Frank Martin, and Clayton Cowles, Thanos is dying and looks throughout the galaxy for a cure to the disease that's killing him. While Thanos quests across the universe, Nebula, Starfox, Tryco Slatterus, and his secret Inhuman son, Thane, try to get to the Mad Titan so they can put him down for good. However, Thane steals the Phoenix Egg and as he's about to free it, Nebula shoots and kills him, as she doesn't want anyone getting its immense cosmic powers. However, Death resurrects Thane with the powers of the egg, making Thanos' son the new Phoenix.
When Thanos returns to the Black Quadrant, one of the planets he ruled, he finds it completely void of life as Thane and Death watching him. Death urges Thane to unleash his new powers upon his father. Death tells Thanos that she's finally found someone worthy for her love (ouch), and watches Thane blast him with the Phoenix Force. As Thane sits on his on his new throne in Thanos' former kingdom, Thanos gets up from his attack, realizing that he has no powers and is simply mortal.
Later issues reveal Thanos is alive but completely depowered. In order to survive, he resorts to hunting rodents and eating them. Thanos is shown sick, weak, and defeated - as he's even beaten up by a pair of scavengers who don't believe he could possibly be the Mad Titan. The group that previously traveled with Thane realizes that Thanos is the key to stopping his maniacal son, who is now taking after his father and destroying planets with ease with his new Phoenix powers. Nebula in particular doesn't take kindly to Thanos, as she hits him hard enough that he bleeds. Without his powers, Thanos is a shell of his former self.
However, Thanos eventually gains his power back via the God Quarry, after completing a series of trials that make him remember who he truly is. Thanos and Thane battle and once the latter is stripped of his Phoenix Force powers via the Coven, and the supervillain throws his son into the God Quarry, turning him into stone as he's trapped inside the cosmic realm. It's a tough end for Thane - and not totally unlike Gamora's death in Avengers: Infinity War.
Ultimately, a powerless and desperate Thanos was created after Thane unleashed the full Phoenix Force at him. However, Thanos is far too smart to stay weak for long, and with the help of a magical realm, he gained his powers back and left his son (and Death) in the God Quarry. The arc proved that Thanos won't ever stay mortal for long. His power is inevitable.
