Thetis was a beautiful sea-nymph who was daughter of Nereus (one of many nautically-named Nereïds), married to Peleus and mother of Achilles. The reason for her marriage is not stated in the Iliad - later sources say she was fated to bear a son stronger than his father, and thus Zeus reluctantly rejected her and made her marry a mortal. The wedding of Peleus and Thetis was attended by all the gods, including Strife (Eris) who wasn't invited, but came anyway - with the golden apple inscribed "for the fairest". The goddesses Hera, Athena and Aphrodite quarreled over the apple of discord, and a contest was held - judged by Paris. In the Iliad Book 1 she persuades Zeus to help Achilles after his treatment by Agamemnon (she's done him a favour in the past), she brings all her sisters to mourn Patroclus' death, and in Book 18 she persuades Hephaestus to make new armour for her son.