Anvil

It was called Scimitar when the first Assassin's Creed shipped in 2007 and was renamed Anvil ahead of the sequel. Prince of Persia (2008) used it as well. Later generations went by AnvilNext.

Its speciality is dense cities: hundreds of NPCs reacting to the player and an animation system that lets you climb almost any façade. That takes aggressive LOD, level streaming and cheap pathfinding for the crowd. Ubisoft runs several in-house engines side by side — Massive's Snowdrop is another — and Anvil has never been licensed out.