It is not that common for a goddess to be overshadowed by her mortal offspring, but that is just what happened to the Nereid Thetis, as she was mother to Achilles.
Thetis as a Nereid was a sea nymph, and one of the fifty daughters of Doris and Nereus, a pre-Olympian couple of sea gods. Thetis was able to change her shape as she desired, as well as to see into the future, although in the earliest stories Thetis is simple shown as a companion to Poseidon. Being one of the companions of Poseidon though brought her to the attention of both Zeus and his brother Poseidon, and both sought to mate with her. She spurned their attentions though but for once Zeus did not continue his attempts, and this was for the simple reason that a new prophecy had been told by Themis. It stated that the son of Thetis would be greater than his father. Zeus of course could not allow a son to be greater than himself, as this would be a serious challenge to his position. Equally though he could also be threatened if Thetis was married off to another god.
Zeus came up with a solution though and decided that if Thetis was married off to a mortal, then even if the son was greater than his mortal father then it that son would not be a threat to him. Poseidon was given the job of finding a suitable mortal, and the sea god decided on Peleus, the son of the king of the island of Aegina, Aeacus. Thetis though had other ideas than to be married off to a mortal, and so refused the mortal's advances.
Zeus though was not to be thwarted in his plan and so sent the centaur, Chiron, to advise Peleus on how to make Thetis his wife. Peleus was told to ambush the Nereid and to bind her tight as she would attempt to change shape to escape. This Peleus did, and Thetis relented and agreed to become his wife. It was a celebrated wedding one to which a number of gods attended when the event occurred on Mount Pelion. During the wedding feast Apollo serenaded the guests with tunes played on his lyre, and the Muses sang their beautiful songs.
The result of the union between Thetis and Peleus was the birth of a son, Achilles. Thetis though is upset to find that her son is mortal and tries to make him into an immortal. Most stories tell of this event as being one where Thetis covers Achilles in ambrosia and then places the child in a fire to burn away the mortal parts of the baby. This process though is interrupted by Peleus who is of course horrified by the spectacle and Thetis drops the baby and flees never to return to Peleus again.
The more commonly quoted story has been overtaken by a more famous one. In this version Thetis dips the new born Achilles into the River Styx, keeping hold of him by his heel, thus his body is made invulnerable, apart from the heel that had not been immersed in the river.
There are several other stories of Thetis before the Trojan War begins, although they are not widespread ones, and are rarely told by multiple sources. There is in fact only a single source that tells of Thetis aiding Zeus against an uprising by the other Olympian gods. This she did by calling a giant, Briareus, to protect Zeus from Hera, Poseidon and Athene. Thetis was also said to have rescued Hephaestus and Dionysos from the sea when both had been thrown out of Olympus. Thetis was said to have given both refuge until they were no longer fearful of retribution.
Thetis returns to the life of Achilles when the Trojan War breaks out. A prophecy had been made abut Achilles, it was said that he would either live a long and dull life, or would have a shorter glorious one. Thetis wanted the prior for her son, and so hid him away as the armies gathered for the war. Achilles was disguised as a girl in Lycomedes' court. It was though a disguise that was easily penetrated by Zeus who visited the court. Achilles was tricked into showing himself for his true self by choosing between armour and female finery.
Achilles therefore was to be a member of the Greek army, and Thetis was left with few options but to try and protect him as best she could. This she did by asking for returned favours from Hephaestus and Dionysus. As a result Achilles was soon the owner of a shield and armour crafted by the smith of the gods. Thetis though could do little to aid in the fighting, but for a long time it looked like her son would need no help, as he proved himself to be one of the greatest of all of the Greek soldiers. It was though as the prophecy had said, Achilles' life was to be a short and glorious one, as Achilles was killed by Paris.
Thetis then came up from the ocean with the other Nereids in order that his body could be collected. It may seem strange that Thetis did not ask Zeus to resurrect her son, but she did not, nor did she allow his spirit to go to Elysium, the resting place of heroes, but instead had it transported to the White Island. White Island became one of the central worship places for the fallen Achilles in ancient Greece.