Thetis is most famously the mother of Achilles, the great (almost) invincible hero whose only weakness was a small spot on his heel (known now and forever as the “Achilles Tendon”).
She wasn’t just the mother of a hero, she was also the queen of the Nereids, the fifty water-nymph daughters of Nerius and Doris. She was also a prophet and could see omens of the future, much like the Oracles of Delphi.
It is said that Zeus himself desired her, but she rebuffed his advances
time and time again. That is until Themis, one of the goddesses of
divine order and custom herself revealed that the Fates had decided that
Thetis would one day have a son who was greater and stronger than his
father. Being the egomaniac and control freak that he was, Zeus
couldn’t stand the thought of having a more powerful son, so he set the
Nereid Queen up with the mortal Peleus, one of the Argonauts.
Relieved, Zeus threw the couple a huge wedding that all of the gods attended. Together the newlyweds gave birth to Achilles who turned out to be one of myth's greatest heroes and led the Greeks to victory in the Trojan War.