Skill Up & Ascension MaterialProof that you've been acknowledged as a hero. Many possess it, but they aren't conscious of it so they forget.
Okeanos: Pirate Ship (Free Quest) is the best AP-saving & time-saving location to farm Hero's Proof (Proof of Hero). You'll get around 6x Hero's Proof by spending 120AP.
Here is a list of servants that need Hero's Proof to perform Ascension or Skill-Up.
Note: No Bond CE's here.
5 Star Servants Tier List
Best Servants [★4]
Christmas event [JP 2016 Rerun]
FGO Reroll Tier List
Holy Night Supper
Piece of Year 2030