Ramírez is depicted as a sorcerer and the Quickening is now said to be a "kind of" magic he uses to create a bond between him and MacLeod that not even death can break. Captured by Katana's forces, Ramírez, MacLeod and the other prisoners are exiled to Earth, somehow becoming immortal in the process.
The Quickening is described as a form of magic that binds Ramírez and MacLeod together, making it impossible for them to be separated by death.  Their imprisonment and subsequent immortality on Earth are consequences of this bond.