So this one takes the cake, easily. Remember the demoness who destroyed Mjolnir? You know, this woman?
That's right, Odin's actual firstborn? Hela, the goddess of death? Well, long before Thor or Loki entered the picture and started their sibling shenanigans, there was Hela. And this was also long before Odin was a peaceful king. This was a dark time in the Nine Realms of Asgard. She and Odin unrelentingly assaulted realm after realm, seeking to build a glorious empire. Then Odin had a change of heart. He realized the carnage he was causing, and saw that Hela's appetites for destruction and death were becoming too insatiable to control. So not only did he build peace treaties with the realms he formerly conquered, but he banished her away using his godly power, in Hel (not the biblical Hell. Norse Hel, the underworld.) AND he covered up the fact that she even existed. He acted like Hela wasn't even a part of his life. It's bad enough that he tried to conquer and be a bloodthirsty warrior king and did so with his daughter, but he covered up the fact that his daughter was even born. Hid it from history, and moved on. And didn't even give it a second thought. Imagine if your own parents abandoned you somewhere and decided to act as though they never had you. How would you feel if you ever found that out? It's pretty dismal and dark, if you ask me.
Thanks for reading: What is something from the Marvel Universe that is really kind of dark, but gets ignored?, Sorry, my English is bad:)