And the rest of Amumu tales:
One oft-told story links Amumu to the first great ruling family of Shurima who succumbed to a disease that corrupted flesh with hideous speed. The youngest child, Amumu, was quarantined in his chambers and befriended a servant girl who heard his cries through the walls. She regaled the lonely heir with courtly news and stories of her grandmother's mystic powers.
One morning, the girl brought word that Amumu's last remaining brother had passed away, making him Emperor of Shurima. Saddened that he had to bear this news alone, she unlocked his door and ran inside to comfort him face to face. Amumu threw his arms around her, but as they touched, he fell back, realizing he had condemned}} her to the same terrible fate as his family.
Upon the girl's death, her grandmother placed a twisted blight on the young emperor. In her mind, Amumu had as good as murdered her kin. As the curse took effect, Amumu was trapped in his moment of Despair.png suffering like a locust ensnared in honeyed amber.
A second tale whispers of another crown prince, one given to bouts of petulance, cruelty, and murderous vanity. In this telling, Amumu was crowned Emperor of Shurima at a young age, and convinced he was blessed by the sun, he forced his subjects to worship him as a god.
Amumu sought the fabled Eye of Angor, an ancient relic entombed in a gilded crypt, said to grant eternal life to whoever looked upon it with an unflinching heart. He hunted the treasure for years with a host of slaves who carried him through labyrinthine catacombs, sacrificing themselves to traps so the emperor could continue without hindrance. Amumu finally reached the cyclopean golden archway, where upon dozens of his stonemasons labored to breach the sealed door.
As the young emperor rushed within, determined to look into the Eye of Angor, his slaves seized their chance and sealed the stone doorway behind him. Some say the child emperor endured in the darkness for years, his loneliness driving him to insanity and causing him to claw at his own skin, which he was forced to wrap in bandages. His life was extended by the power of the Eye as he meditated on his past transgressions, but the gift was a double edged sword, for he was cursed to remain forever alone.
When a series of devastating earthquakes shattered the foundations of his tomb, the emperor escaped with no knowledge of how much time had passed, seeking to undo the suffering he had caused in life.
Story by Khaldun
TL:DR, Amumu's lore is a bunch of different stories and tales, it's never said what he really is, the only thing confirmed that he is a mummy with a curse.
The Shopkeeper is Old and Outdated, Riot probably kept those quotes to make him more of a mystery like how "The Twisted Treeline altars do not have quotes dedicated to Evelynn out of a desire to not reveal much about her."