Greater Mark of Justice was a spell cast by Lord Shojo on Belkar while in custody. The Mark required that Belkar should not kill anyone inside a town or village and should not distance himself from Roy, otherwise the Mark will be activated. Roy could also activate the mark by saying a particular magic word that only he knew ("Squiddley-doodlefluffer").
The Mark of Justice forced Belkar to do things he would not do otherwise, like keeping close to Roy and help recover his body.
When he killed the Oracle, the spell was activated; because the Oracle had foreseen this murder, and in order to take his revenge in advance, he had founded the village of Lickmyorangeballshalfling in the Sunken Valley. When Belkar killed the oracle, the Mark triggered, causing him to fall ill, vomit, and have hallucinations. However on leaving the Sunken Valley, the spell that kept visitors forgetting, made Haley and Celia forget about the Mark, and believed that Belkar had simply a disease.
Eventually Belkar was healed by the Cleric of Loki in Greysky City. The deactivation passphrase was "Evolve or die!"
The spell is based on an actual D&D spell, Mark of Justice.