Poison the Well makes every Blighter start fights with less health. It is one of the most direct ways the Ringleader branch weakens the enemy.
Poison the Well is entry 14 of 17 in the Ringleader branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £5,000, 50 Chemicals. Before Poison the Well unlocks you need Carriage Sabotage.