Question: In Greek mythology, Pyrrhus is the grandson of what immortal nereid?

Context paragraphs:
Pyrrhus III: Pyrrhus III, king of Epirus, was a son of Ptolemy and a grandson of Alexander II.  He ascended to the throne in 235 BC.  Pyrrhus III was assassinated, and he was then succeeded by his cousin Laodamia, or Deidamia (daughter of Pyrrhus II) who was the last of the Phyrrhus line.  With his death, the male line of the royal family of Epirus became extinct.

Achilles: In Greek mythology, Achilles ( , ; ] ) was a Greek hero of the Trojan War and the central character and greatest warrior of Homer's "Iliad".  His mother was the immortal nereid Thetis, and his father, the mortal Peleus, was the king of the Myrmidons.

Arethusa (mythology): Arethusa ( ; Ancient Greek: Ἀρέθουσα ) means "the waterer".  In Greek mythology, she was a nymph and daughter of Nereus (making her a Nereid), who fled from her home in Arcadia beneath the sea and came up as a fresh water fountain on the island of Ortygia in Syracuse, Sicily.

Cleodaeus: In Greek mythology, Cleodaeus was one of the Heracleidae, a grandson of Heracles.  He was the son of Heracles's eldest son Hyllus and Iole of Oechalia.  He became father of Aristomachus, who led the third attempt to capture Mycenae and failed.  He also had a daughter Lanassa, who married Neoptolemus and had by him several children, including Pyrrhus.  Cleodaeus had a heroon at Sparta.

Psamathe (Nereid): Psamathe (Greek: Ψάμαθη, from ψάμαθος "sand of the sea-shore") was a Nereid in Greek mythology, i.e., one of the fifty daughters of Nereus and Doris.  The goddess of sand beaches, Psamathe was the wife of Proteus and the mother of Phocus by Aeacus.

Nymph: A nymph (Greek: νύμφη , "nýmphē" ] ) in Greek mythology and in Latin mythology is a minor female nature deity typically associated with a particular location or landform.  Different from other goddesses, nymphs are generally regarded as divine spirits who animate nature, and are usually depicted as beautiful, young nubile maidens who love to dance and sing; their amorous freedom sets them apart from the restricted and chaste wives and daughters of the Greek "polis".  They are beloved by many and dwell in mountainous regions and forests by lakes and streams.  Although they would never die of old age nor illness, and could give birth to fully immortal children if mated to a god, they themselves were not necessarily immortal, and could be beholden to death in various forms.  Charybdis and Scylla were once nymphs.

Pyrrhus (Royer): Pyrrhus is an opera by the French composer Joseph-Nicolas-Pancrace Royer, first performed at the Académie Royale de Musique (the Paris Opera) on 26 October 1730.  It takes the form of a "tragédie en musique" in a prologue and five acts.  The libretto, by Fermelhuis, is based on the Greek myth of Pyrrhus, son of Achilles.

Theoi Project: The Theoi Project (also known as Theoi Greek Mythology) is a digital library website about Greek mythology and its representation in classical literature and ancient Greek art.  It serves as a free reference to the gods ("theoi"), spirits ("daimones"), mythological creatures ("theres") and heroes of ancient Greek mythology and religion.  Established in 2000, the website contains over 9,000 pages and 9,000 images.  The website also has a library of classical literature on the theme of Greek mythology, including the works of many of the lesser-known poets.

Dynamene: In Greek mythology, Dynamene (Ancient Greek: Δυναμένη ) was a Nereid or sea-nymph, one of the 50 daughters of Nereus and Doris.  Her name, a participle, means "she who can, the capable one."  She, along with her sister Pherusa, was associated with the might and power of great ocean swells.  She is mentioned in Hesiod's Theogony.

Olympias II of Epirus: Olympias (in Greek Ὀλυμπιάς, ] ; lived 3rd century BC) was daughter of Pyrrhus, king of Epirus from his first wife Antigone.  She was the wife of her own paternal half-brother Alexander II.  After his death she assumed the regency of the kingdom on behalf of her two sons, Pyrrhus II and Ptolemy; and in order to strengthen herself against the Aetolian League she gave before 239 BC her daughter Phthia in marriage to Demetrius II, king of Macedonia.  By this alliance she secured herself in the possession of the sovereignty, which she continued to administer till her sons were grown up to manhood, when she resigned it into the hands of Pyrrhus II.  But the deaths of that prince and his brother Ptolemy followed in quick succession, and Olympias herself died of grief for her double loss.  Such is Justin's statement: according to another account Olympias had poisoned a Leucadian damsel named Tigris, to whom her son Pyrrhus was attached, and was herself poisoned by him in revenge.

Supporting paragraphs: Pyrrhus (Royer) Achilles
Answer: Thetis