The old mother was a legendary kraken in the sea of thieves, also known as the "Mother 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, where the only inhabitans of the sea of thieves at this point in time.
At this time, the ancients worshipped a huge and ancient kraken, called the old mother, wich was said to be so huge, that a
large vessel could go trough its ribcage, 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 refered to the beast as the "queen of all krakens". The ancients saw the old mother as the bringer of death, and
they brought sacrifces to the old mother and other krakens, in the hopes to statify their hunger, but what ended up
happing 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 the Ancients and the Merfolk made up a
plan to construct mystical, unbreakeble chains to capture the old mother, and bringing a stop to her terror on the sea. When these chains where constructed, the
Ancients and the Merfolk waited until the old mother would fall asleep, and then the captured her, chaining her down to the oceanfloor with the unbreakenble chains.
After some time, the old mother died of starvation, and her reign of terror had ended.
Years after this, the pirate lord Ramsey Sing heard the stories about the old mother and the unbreakeble chains, and he actuelly found these magical chains,
wich he used to craft the chests that can now be found all over the sea of thieves.