Another reason the nereid Thetis and Peleus, the mortal king, were well known was their only child who reached maturity, the Greek hero Achilles. He was their seventh son, but all the others before him died, because Thetis tried too hard to make them invulnerable. She did the same to Achilles, too, by keeping him over a fire by night, so as to burn away the mortal elements of his body, and anointing him with ambrosia by day.
One night, though, Peleus finally discovered what she was doing. In horror, he gave a scream, and Thetis threw the crying baby to the ground. In fact, Peleus had just saved his son's life. Angry, Thetis left her husband for good and returned to the Nereids, but she continued to keep an eye on her son. Peleus gave the child to Chiron, the wise Centaur, to be his pupil.
Another version narrated that, in order to achieve invulnerability, the nereid Thetis dipped the baby in the waters of the river Styx, but she held him by the heel, and that's why the heel remained the only vulnerable part of his body. (Until today, the expression "Achilles' heel" refers to a person's weak point.)
As Chiron's pupil, Achilles grew up as a very brave and talented young man. But when it was clear that the war of Troy was going to begin soon, the nereid Thetis, who had the gift of prophecy, came by and took her son to the court of king Lycomedes. She dressed him in girl's clothes and introduced him as Pyrrha, Achilles' sister. In the end he accepted this farce, because it was the only way for him to get closer to Deidamia, the king's daughter.
But the cunning Odysseus managed to blow out his disguise, and Achilles had to go to war. From then on, his mother tried to help him, knowing that he was going to die in war.
Thetis gave him a servant, Mnemon, whose job was to remind Achilles not to kill one of Apollo's sons, because that meant he would die at the god's hand. Unfortunately, Mnemon didn't manage to warn the hero, who killed Tenes, and so his fate was sealed.
Thetis had warned Achilles he shouldn't be the first one to set foot ashore, because there was a prophecy saying that the first one to touch the ground would be the first one to die in war. On this one, Achilles managed to obey his mother.
When Agamemnon had to give back Chryseis, the daughter of a Trojan priest of Apollo, he decided to take instead Achilles' beloved slave, Briseis. The hero was so offended by this, that he retired to his tent and he took no part any more in the military actions.
Achilles asked his mother to go to Zeus and ask him to avenge his honour. In doing this, he mentions that Zeus is in debt with Thetis, because she was the only one who helped him when Hera, Poseidon and Athene put him in chains. The Nereid Thetis decided to call one of the Hekatonkheires, Briareus, who managed to chase away the other gods, thus freeing Zeus.
So Thetis went to Zeus and kneeled near his throne. She put one hand on his knees and with the other hand she grabbed his chin and begged him to avenge this humiliation. That's why the Greeks were severely defeated on the battlefield, until they had to beg Achilles to return in battle.