A character in Virgil's Aeneid, the son of Hyrtacus, appearing solely in Book 9, where he and his friend Euryalus set out from the besieged Trojan camp by night to find Aeneas, but are caught in the Latian camp, where they kill many Rutulian heroes before themselves being killed, caught in flight by Volscens.
The tale of Nisus and Euryalus takes up over half of Book 9. It is a story of loyalty between friends - a very important stoic value, and the waste of youth in war, one of Virgil's main themes of the Iliadic half of the epic, encompassing books 7 to 12. Virgil's tribute to them after the incredibly moving account of their deaths explains the raison d'être of his account - to give glory to two friends who encompassed much of the Stoic philosophy.
O happy friends! for, if my verse can give
Immortal life, your fame shall ever live,
Fix'd as the Capitol's foundation lies,
And spread, where'er the Roman eagle flies!
Coincidentally, as Webster 1913 has so lucidly and concisely told us, nisus comes from the latin verb nitor, meaning 'I struggle'. It is unclear whether the etymology of Nisus' name is related, but considering the strong stoic influences of the book, and indeed the whole work, it seems likely.
Translation by John Dryden. The text is in the public domain and it can be found, in its entirety, on this site, at Aeneid.