Trap of Argalia

Down with a Touch! The second cheat Noble Phantasms of Rider of “Black.” It is a golden lance that originally belonged to Argalia, a knight of Cathay. With this magic lance that knocks down all those that it touches, Argalia was victorious against all the knights who challenged him in jousting matches, including Astolfo.
However, one knight didn't surrender after being knocked off his horse and drew his sword. Reluctantly, Argalia fought him with his own sword without using his lance, but he couldn't match the knight and ran away in the end. Argalia had trusted in the power of his magic lance too much, and so he had promised that the one who beat him would be presented his older sister Angelica (the beautiful woman who was the cause of Roland going mad later on).
Angelica also understood how bad the situation was once she learned of her younger brother's defeat and so disappeared into hiding, and while the entire jousting contest was in total chaos, Astolfo, who had broken his own lance, noticed Argalia's lance, remarking “Ah, so there was a lance lying around here,” and he sto… silently borrowed the lance.
Naturally, Astolfo won match after match in jousting contests afterwards.
The Noble Phantasm “Trap of Argalia” is a lance, but its actual power is very low. However, when an opponent is touched by the front tip of the lance, they will be forced into a state of falling down—in other words, their legs are forcibly changed into spiritual form. Of course, this is only temporary (after all, they are only forced into a state of “having fallen down”), and they can recover in a short period of time, but just as demonstrated in the third volume, it possesses extremely strong compelling force that can “even topple a giant.”
Incidentally, after Astolfo managed to survive through several battles with this lance, he entrusted this lance to the female knight Bradamante, the cousin of Roland. Naturally, Astolfo freely gave away such a precious lance because he didn't know about its ability at all. As for how he kept winning in jousting matches, he apparently thought, “I 'awakened' the power unconsciously sealed inside me!” How awful.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia