The answer to this question is clearly shown on the LOTR: Fellowship of the Ring... It is when Arwen cast the Water Horse Spells on the Wraiths... After that Frodo began to fade then "HERE'S THE ANSWER", Arwen prayed to give her gift to Frodo to endure the poison from the Morgul Dagger....

The answer to this question is clearly shown on the LOTR: Fellowship of the Ring. After Arwen cast the Water Horse Spells on the Wraiths, Frodo began to fade. Then — here's the answer — Arwen then prayed to give her gift to Frodo to endure the poison from the Morgul Dagger.