Noun
wedge - A piece of material with a thick end that tapers to a thin edge, used to split, support, or secure things; also something that forces separation or division between people or groups.

The disagreement drove a wedge between the two longtime friends, and they really didn't enjoy spending time together anymore.
ringleader - Someone who is the main influence or motivator behind a particular action or event, especially a disruptive or controversial one.

The police managed to arrest the ringleader of the gang, who had coordinated the series of robberies across the city.