About the Sherlock Holmes Devils Dust MMPB

Mrs Hudson begs Holmes and Watson to help a fellow landlady whose lodger has been found poisoned, and who the police suspect of the murder. The companions discover that the lodger, a mining engineer recently returned from Africa, was on the run. His last act was to scrawl 'Devil's Dust' in his own blood. What was he trying to convey, and what is the role of hunter Allan Quatermain, who visited the man just before his murder?