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.
The spell is based on an actual D&D spell, Mark of Justice.