Methos shifted uncomfortably. "Immortals, my kind of Immortals, each have a Quickening - a source of energy, and the source of Immortality. It can be passed from one Immortal to another if they're present when an Immortal has their head cut off. Usually why we fight each other. The older an Immortal is, the more Immortals he's killed, the more powerful the Quickening."
He finished the first beer and opened a second before continuing. "Imagine what it would be like for someone to take the Quickening of the oldest Immortal."
There was a reason he was paranoid, and had been very careful to find a weapon before he bothered with clothes when he woke up without his sword or his gun.