The Unfinished Tale of Amumu, the Sad Mummy (Theory)
Shout out to Riot's Music and Art team for creating such an amazing music video for one of the most pitiful characters...
...but the story does -not- end in depression of solace, and I'll tell you why. Let's begin:
Amumu, the Sad Mummy, was once a proud emperor of Shurima. People groveled before him out of affection, worship, or tyrannical fear; none could tell. Many had mistaken the pharaoh for a child, but was actually a yordle. On the final hieroglyph showing Amumu's legend, it shows his death;-- those close to him mourned as others celebrated. No one could tell if he was benevolent or malicious.
According to his official lore, his curse is of loneliness that withers those who are near him. As seen in the story of the young man who first finds Amumu in his funeral tomb (point, though: the young man dropped his lantern 
). Amumu cried, pleading for the man to "be his friend", and grabbed his arm. The withering curse afflicted the man's hand, and slowly crawled up his arm. You can probably guess what happened after that point.
Amumu's journey is confirmed to have started in Shurima. From what it looks like in the "Curse of the Sad Mummy" video, it looks as though he woke up before the events of Ascension. After the Shurima scene, he ended up in a tropical forest region (and considering where Wriggle's Lantern was recovered...), it'd be safe to assume he tried to follow the man, and ended up in the jungles of Kumungu. (Unbeknownst to Amumu, he was running from him.)
"For many years, young Amumu traveled across the land, determined to make friends, if only they would understand."
Amumu had traveled throughout Valoran and to Bilgewater and Ionia looking for people who could assist him in his curse, but was unable to find any help. At this point, it seems he would have met his only known friend: The Dark Child, Annie. He traveled back to southern Valoran into the Voodoo Lands and met Annie in the Grey Council's small hamlet. Whilst others shut him out, Annie may have taken intrigue in her mummified acquaintance, seeing as she herself is the outcast Dark Child. There is hope for Amumu through Annie.
"But when Amumu stood upon the ledge of home, his hope would disappoint him, and he would remain alone."
After traveling across the continent, he ended up in what looked like a Yordle villiage, most likely Bandle City. When entering the city, he noticed everyone was like him (short, eccentric, curious, etc.), but still people condemned his being, and shut him away. Alas, his tantrum took the best of him, and confirmed that he would be lonely forever unless his curse was dispelled.
Again, however, he has Annie at his "side" (about 5 feet minimum distance), and he has yet to return Shurima where he was reborn. Could Nasus or Azir hold some answers to his condition? Could his curse have been caused by Nasus's withering ability? Why is Amumu undead? Many questions remain, as Amumu's story is left untold...
UPDATE: Some other lore theorists have suspected that curse was actually a spell cast on Amumu (by his parents, a magician, etc.) to make his loyal subjects love and praise him. Because of this, he must pay for it in the afterlife.... Interesting theory!
UPDATE UPDATE: An anonymous community member posted a continuation of the lyrics online, which gives a desirable ending to the tale, as his champion page proves it wasn't the end.
"Amumu left the town to anguish in the damage he had wrought. He settled in a swamp knowing there was nothing left to rot. "There he hoped to stay forever, in solitude and gloom. In doing this, he prayed no one else would meet their doom. "There one year he heard a noise not familiar to that place-- A little girl who stared upon him with a smile and wonder on her face. " He tried to run he tried to hide, but she thought it was a game. When she caught him and to his surprise when she gazed upon his face She did not scream she did not die, she was completed left un-phased. "A witches child, her name was Annie, the curse she was immune. When she laughed, when they played, it made his heart beat as if anew. "For his search was over, Amumu finally met a friend. Never was he sad again, his curse was at its end."
Hey guys, here's a link to some of my previous story theories:
Comment down below if you want to see more!
is Azir's 
son. Azir had enemies in high places and they placed a curse on his only first born that he would be alone for all of eternity. Azir has magic and it has been handed down through his blood line. In the music video you can see him come out of a sarcophagus. He must have been baried alive.