Basicly my idea is a special terrain type called perhaps:
Dirty Water
Stagnant Water
Leech Pond
I dunno - either way it is a disgusting, most unpleasant form of water terrain. Wading through it decreases the happiness of most creatures.
But its key distinguishing feature is that creature wading through have a chance of being latched onto by hostile neutral 'leeches' which latch onto a creature and lower their health regeneration, efficiency and accuracy until they are gotten rid of. This can be done in the temple, by being set aflame or by being healed. The leeches also have hit points and take identically damage to the creature if he is attacked - since their hitpoints are lower than the creatures (they scale) they will often drop off through the course of combat aswell. But beware - this will not always work because leeches have high health regeneration as they have the victim creatures body to support itself as well.
A creature with leeches will have leeches attached to his model and a black hostile neutral creature health star above his own showing its health and level.
LEECH
Health - Scales to attached creature (but always 50% lower)
Armor - Low
Defense - Low
Accuracy - Medium
Attack - None (Except debuffing)
Health Regen - High