From Fate/Grand Order Wiki
"Ominous bones wielding curses."
This is crowd-sourced information from the FGO Item Drop Rate Sheet. Observed rate may vary depending on the player's luck. Rates are only as accurate as the sample size of drop information available. Rates may change and become out of sync between the sheet and the wiki; they are only updated on the wiki when actual people, like you and I, edit the relevant quest and item pages. It is also an option to obtain items from Events instead, when they may appear as Event rewards or become available as loot from Event quests.
Used for Ascension & Skill Enhancement by
15 & 30