RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Holy table napkin
The Old School RuneScape Wiki also has an article on Holy table napkin.

The Holy table napkin is a quest item obtained in the quest The Holy Grail. It is given to the player by Sir Galahad. The player cannot obtain another napkin after the quest has been completed.

The magic whistle found at several spawn locations within the Fisher King Realm. Obtaining a whistle means that the napkin may be safely discarded if the player keeps the magic whistle which they found during the quest.