The old mother was a legendary kraken in the sea of thieves, also known as the "Queen of all krakens."
Backstory
Long before the first pirates came to the Sea Of Thieves, there lived multiple tribes of people called "The Ancients" on the islands. They, together with the merfolk, were the only inhabitants of the sea of thieves at this point in time.
The ancients worshiped a huge and ancient kraken, called The Old Mother, which was said to be so huge, that a large vessel could pass through its rib cage without touching it. This kraken is said to be the mother of all krakens we see today in the sea of thieves, and the ancients also referred to the beast as the "queen of all krakens". The ancients saw the old mother as the bringer of death, and they brought sacrifices to the old mother and other krakens in the hopes to satisfy their hunger. But what ended up happening was a increased kraken population that would terrorize the ancients and the merfolk.
At some point the ancients and the merfolk met at a underwater cave to discuss this problem. There, they planned to construct mystical, unbreakable chains to capture the old mother, and bringing a stop to her terror on the seas. Once constructed, the Ancients and the Merfolk waited until the old mother fell asleep, captured her, and chained her to the ocean floor with the unbreakable chains. After some time, the old mother died of starvation, and her reign of terror had ended.
Years later, the pirate lord Ramsey Sing heard the stories about the old mother and the unbreakable chains, and when he found these magical chains, he used them to craft the chests that can now be found all over the sea of thieves.