User blog comment:HazelQuill7445/Awesome fan art/@comment-27144409-20171007163804/@comment-25936766-20171007170041

Well, leaving aside how Servants are supposed to be OP - with the only real question being "how much" - Noble Phantasms are basically a combination of a Heroic Spirit's weapons and/or abilities and the imagination of humans.

They embody the "ultimate mysteries" of a Heroic Spirit's own existence, including both historical facts and fantasy that have both been engraved in history. They can be physical objects or abstract concepts and abilities.

For example, Vlad the Impaler has Kazikli Bei, basically the ability to summon stakes, representing how he loved impaling people. But he also has the Legend of Dracula, which turns him into the vampire that has tarnished his name in exchange for his sanity.

This makes them a last-resort of sorts, since they provide so much info on the Servant, just looking at them is enough for everyone to know "This guy is X", and plan accordingly.