Andreias Amarantos

is the Noble Phantasm maintaining Achilles' gift of immortality from having been blessed and exalted by the Olympian Gods, protecting him from all ill will and killing intent.Fate/Apocrypha - Volume 1Fate/Apocrypha - Volume 2
Although his mother, the goddess Thetis, had wanted to make him into a fully immortal god by warming him within holy flames to extinguish his human blood, her husband, Peleus, opposed it because it would "destroy him as a human", so only one part of him, his heel, remained human after he stopped the process midway. His heel is the sole, vital weak-point for the otherwise invincible hero, but he is not afraid to attack opponents with it should it be required. His immortality kept him from receiving any wounds in life until his weak-point was exposed during his fight with Hector, and later utilized by Paris to kill him under the guidance of Apollo. The act of having his heel hit negates his immortality, allowing for his heart to be pierced with numerous arrows as he made his last charge against the Trojan army in life.
Any type of attack against him is nullified, including physical damage, the "normal attacks" of Servants, and even great Noble Phantasms like the A+ ranked Balmung. It cannot be pierced by power alone, but it does have three weaknesses. The first is that only those who possess the blood of a god within their veins are permitted to harm him. It can only be fully negated by those with possessing the Divinity Skill equaling or surpassing Rider's C-rank Divinity, so those possessing D-rank Divinity will have their attacks reduced to 75% of their total damage and those of E-rank Divinity will have their attacks reduced to 50% of their total damage.
 Those without Divinity but in possession of Divine Constructs can deal damage so long as the Divine Construct is being used to attack, and the damage will be based upon the weapon's rank in the same way as Divinity. With only one member able to harm and defeat Achilles, it can be said that without Chiron, the Black Faction cannot win the Great Holy Grail War. Siegfried, with no Divinity, is unable to give him a single wound in drawn-out combat, while Chiron, with C-rank Divinity, is able to draw blood with a simple arrow. The second weakness is that his invincibility does not apply to acts that display friendship. The act of being bitten by a vampire is an act meant to make him into a comrade rather than kill him, so his body will respond by making part of himself weak in response to that request. The third is that damaging his heel will negate this blessing, causing him to lose his immortality as well as reducing his speed by approximately 70%.Fate/Apocrypha - Volume 5: "踵の矢は既に無い。だが、既に実感として理解できる。己の“不死身の肉体”は引き剥がされた。踵は急所でも あり、鍵でもある。あらゆる攻撃を跳ね返してきたこの肉体は、相手が 『神性』 スキルを所有しているか否かは無関係になる。自慢の健脚も、しばらくは七割減というところか。"
In Fate/Grand Order, it is treated as a Skill in gameplay.
==References==
vi:
Category:Noble Phantasms
Category:Anti-Unit Noble Phantasms
Category:B rank Noble Phantasms
Category:Continuously Active-Type Noble Phantasms
Category:Armor
Category:Noble Phantasms introduced in Fate/Apocrypha
Category:Noble Phantasms of Achilles