The only deity or demigod not invited to the wedding between King Peleus and Thetis was Eris, the goddess of strife and discord.
Allen
Peleus and Thetis didn't invite Eris to their wedding, so she sent the golden apple that she knew would cause a fight
Eris, personification of discord, threw a golden apple with the words "To the fairest," at the wedding of Peleus and Thetis.
She was a trouble maker.
Eris the spirit/goddess of strife and discord. She was upset because she was not invited to the wedding.
Achilles
Achilles.
If you mean the wedding that happened before the War of Troy that sort of relates to it, then it is Eris. The wedding was between Peleus and Thetis, the parents of Achilles.
It was at the wedding of Thetis and Peleus to which the golden apple "for the fairest" was found which Aphrodite, Hera, and Athena all fought over.
Peleus and Thetis were his parents.
Thetis was Achilles' goddess mother, Peleus his mortal father.