You know not when the first ancient ones rose from the depths of the sea. Massive creatures, reminiscent of whales, yet ancient and terrifying. Your ancestors made the pact with them (or perhaps, more accurately, they made the pact with your ancestors). In the centuries that followed the Aspidochel have served as your people's living ships - the only safe conveyance across the dark and dangerous waters of the Starlit Sea.
Deadly things dwell within, you know, but nestled safely in the bowels of your living ships, you are safe.