[Edi]

To the question of the OP, it is holdover flavor text. There is no assassination attempt by knights. There is a random event where knights try to liberate a province under your control, but it does not require the Chalice to be present.

"A group of knights has unexpectedly attacked our province in an attempt to take control."

Requires Turn 8, Misfortune 2 and middle era. Attack by 2 Knight Commanders, 4d6 Knights and 3d6 Militia.