TMdict

Speartip of the Star Traversing the Skies. The Noble Phantasm of Rider of “Red,” Achilles. To describe it in a single phrase, it is a spear for fighting one-on-one. It creates an arena cut off from all of the surrounding environment and time, which isn’t released until one of the combatants is defeated. In terms of magical theory, it is virtually the same as “Aestus Domus Aurea: Golden Theater of the Flamboyant” that “Red” Saber-san from Fate/EXTRA creates.
It can be called the ultimate contest of pure strength and skill that negates even “flukes” caused by things like divine blessings or luck. Achilles used this when he fought Hector. Furthermore, while he fought barehanded in his fight against Chiron, nothing prevents either combatant from using weapons in this arena.
Also, since Achilles killed the Amazoness Queen Penthesilea with this spear, he cannot use it against women. In the first place, this spear can only be used against “those who Achilles wants to fight one-on-one against.” In other words, if Achilles doesn’t “wish to fight them” and the opponent doesn’t have enough courage and skill to respond to his challenge either, Achilles won’t use this spear. Furthermore, even if Achilles himself wants to fight them, he won’t try to forcefully bring his opponents into the one-on-one arena if they don’t want to. It is a Noble Phantasm with a narrow range of enemies it can be used against.
Also, if Achilles should be summoned as a Lancer, it will gain an additional ability separate from the arena creation ability; it is a power that resembles “Gáe Buidhe: Yellow Rose of Mortality,” which “makes healing impossible when one is wounded by this spear.” He doesn’t have his chariot in the Lancer class, but he can wield this spear with divine speed while dashing around the battlefield with equal swiftness, so he is an unmanageable threat in either class.