Agathodaemon's Natron

Origin
Agathodaimon was an alchemist who created a substance he called 'fiery poison', and judging by his account, was arsenic trioxide, a highly toxic amphoteric oxide. The formula was likely a mix of realgar or orpiment fused with natron in solution.

Effects
When subjected to, a person will experience a fatal and slow working form of arsenic poisoning. The user will feel an extreme burning sensation throughout the entire body, which worsens over time. Small chunks of it can incapacitate men, while whole pieces have been reported to poison villages.