Warning: contains spoilers for X-Men: The Trial of Magneto #5!
The death-obsessed Mad Titan Thanos has had a single-minded obsession with killing all living beings in the universe, in the hopes of finally getting validation and love from Marvel's living embodiment of death, Lady Death. Sadly for him, Wanda Maximoff just revealed that may now be impossible.
Thanos first met Lady Death as a child on his home world of Titan, and although their exact relationship is up for debate, it is clear that the Mad Titan fell obsessively in love with Death almost immediately. Since then, he has schemed to extinguish all sentient life in the universe just to prove his loyalty and love to Death, confirming along the way that at this point he views life as a personal insult. Recently, Thanos has taken over as leader of the Eternals, with hints he intends to eradicate Earth itself. Simultaneously, The Trial of Magneto event has seen Wanda Maximoff murdered and then resurrected in a complex murder mystery.
In X-Men: The Trial of Magneto #5 - written by Leah Williams with art by Lucas Werneck and David Messina - it is revealed that Scarlet Witch's murder was actually an elaborate ploy set up by Wanda herself, so that she could give the mutants a gift that they would otherwise be unable to accept. Wanda roped her father Magneto into the plot, convincing him to kill her and keep the plan a secret for the betterment of mutantkind, since it will allow them to resurrect mutants who died earlier than their technology can reach. However, as part of her efforts to convince Magneto to go through with her murder, Wanda explains she is now functionally immortal. "I can always find my way back from death," Wanda reveals, "...you know death could never grasp me forever - I'm too far out of reach for such trivial things." Apparently assured that death can never really claim her, Scarlet Witch has truly become Thanos' worst nightmare.
It turns out Wanda is completely right, and she truly is too slippery for Lady Death to grasp her. When she dies, Scarlet Witch goes to a magical realm called the Eldritch Garden, where she is eventually able to resurrect herself using her own Chaos Magic.
Thanos would absolutely hate knowing this, because it means that in most circumstances he could never truly eradicate all living things. Not only are Wanda's individual resurrection powers a nightmare for someone who considers even regular life an insult, but her "gift" to mutants gives Krakoa's Resurrection Protocols access to 20 million dead mutants' Cerebro information, which means that Wanda just effectively made every single mutant past or present an immortal being, albeit dependent on the X-Men to bring them back. Thanos considers his own inability to permanently die a curse, but he'd be doubly enraged to learn one of his longtime enemies now commands this incredible power for herself.
Thanos would 100% view Wanda's immortality as a slap in the face, and her efforts to give mutantkind easier access to their resurrection as well, because it is a direct attack on his love's cosmic purpose and his own ideals. Thanos believes himself, and his plans, to be inevitable... but an immortal opponent would certainly take the wind out of those sails. If Thanos is ever truly going to conquer life as he desires, he will first need to deal with the immortal powers of the Scarlet Witch.
