Final answer:
The longbow was the new weapon that exposed the vulnerability of the fully armored knight during the Battle of Agincourt.
Step-by-step explanation:
The new weapon that exposed the vulnerability of the fully armored knight during the Battle of Agincourt and began the end of the fully armored knight was the longbow.
The longbow was a powerful weapon capable of firing arrows with great force and accuracy over long distances.
It was particularly effective against the heavily armored knights, as the arrows could penetrate the gaps in their armor and cause significant damage.