Prydwen (プリドゥエン, Puriduen?) is a treasure owned by King Arthur, said to be a wonder tool able to become both a ship and a shield, or something like that. Stored away in Camelot's treasure vault, Mordred stole it from Artoria during her temporary rule while the latter was away.
Prydwen Tube Riding: Kingly Mood of Reining in Surging Waves! (逆巻く波濤を制する王様気分！, Sakamaku Hatō wo Seisuru Ōsama Kibun!?) is the Noble Phantasm of Mordred in her swimsuit. Bringing forth Prydwen and modifying it into a surfboard because she sought for a surfboard that could withstand her power, she rams the opponent head-first with Prydwen while tube riding, a surfing technique where she runs through the gaps of the waves. Boasting a top-class degree of difficulty among the surfing techniques, she feels exactly like a king that rules the sea, or something like that, while in the middle of performing the technique.