Peleus was the father of Achilles who married a goddess of the sea Thetis. There was possibly an old story that at his wedding Peleus received three magical gifts – a suit of armour which no weapon could pierce, a spear that always came back to you when you had thrown it, and divine horses that were immortal. These three gifts he gave to Achilles when he went to Troy.
This story Homer adapted somewhat for the Iliad. He doesn’t explicitly say that the armour was impenetrable but when Patroclus went out to battle borrowing Achilles armour, before Hector killed him, Apollo stripped him of the armour leaving him naked. Hector then put on the armour and before Achilles could kill him he had to find a part of the body that was exposed. The divine horses were useful but sometimes when a chariot went out, besides the two main horses there was a trace horse, so that even if the two horses pulling the chariot could not be harmed, the trace horse could so that the chariot would not be invincible – else how could you have dramatic tension if something was invincible. The theory is that Homer baulked at the idea of a spear that always returned to its thrower. This was a bit to magical for his tastes so that he used the device of a god retuning the spear to it owner
τὸ δ’ ὑπέρπτατο χάλκεον ἔγχος,
ἐν γαίῃ δ’ ἐπάγη· ἀνὰ δ’ ἥρπασε Παλλὰς Ἀθήνη,
ἂψ δ’ Ἀχιλῆϊ δίδου, λάθε δ’ Ἕκτορα ποιμένα λαῶν.
The bronze spear (thrown by Achilles) flew over him and stuck in the ground but Pallas Athene snatched it up and gave it back to Achilles without Hector realising.