Mortal Kombat series

Mortal Kombat is a series of fighting video games. It is known for having a very high level of violence, blood, and gore. The first game was so violent and bloody compared to other video games at the time that it was one of the games mentioned in the 1993 congressional hearings on video games. This later led to the ESRB rating system. Unsurprisingly, most games in the series are rated M for Mature, which means that the game cannot legally be sold to anyone under the age of 17 in most stores in the United States.
Guest fighters.
Starting with the 2011 video game, well-known characters serve as guests for the games. Due to legal constraints, they only appear only in one game. There are two guests in MK9, four guests in MKX and five guests in MK11.