The first Assassin’s Creed game’s internal development engine was called Scimitar.
Ubisoft Montreal developed Scimitar for the original Assassin’s Creed, using it to support the game’s large historical cities, crowds, climbing systems, and open-world presentation. The engine was built for the PlayStation 3 and Xbox 360 generation and helped the game present a more continuous urban environment than many earlier action games.
Scimitar was later renamed Anvil and became the foundation for subsequent Assassin’s Creed technology. Ubisoft continued evolving the engine, eventually producing versions known as AnvilNext and AnvilNext 2.0.
AnvilNext is therefore a later engine name, not the original internal name. Disrupt was used for Watch Dogs, and Fox Engine was developed by Konami, making both plausible but incorrect alternatives.