Which term best describes an offence that is only recognized for severe crimes, such as murder?

The term that best describes an offence recognized for severe crimes, such as murder, is "principal in the first degree." This term refers to the main perpetrator who directly commits the crime. In the context of murder, the principal in the first degree is typically the person who carries out the act of murder itself.

Accessory before the fact refers to someone who assists or encourages someone else to commit a crime but does not participate directly in the execution of that crime. Accessory after the fact describes a person who helps someone after they have committed a crime, typically to help them evade law enforcement. Public order offences, on the other hand, involve actions that disrupt the peace or order of society and are not necessarily tied to severe crimes like murder.

Therefore, the distinction lies in the role and participation in the criminal act, with "principal in the first degree" being the most applicable term for describing direct involvement in severe crimes.