Leechmonger

The Leechmonger is a creature from the valley of defilement (level 5-1) of Demon's Souls. It is a demon, amalgamated from leeches, creating an abomination beyond that of any of the normal demons in the game. It sits in an arena with a floor filled with polluted water.

Absorption
The Leechmonger has a special attack in which it absorbs a substance from the air (presumably souls of some description) which heal him as a bonus.

Environmental Weaponisation
The Leechmonger is a creature that is made of leeches, so in turn, it can and will throw balls of his own accumulated mass at players whilst the leeches suck health away for 15 seconds. It also uses it's location to protect it from anyone who would usually attack at close range.

Immunity to Poison
The Leechmonger is encountered lurking in a pit full of poisonous scum, yet is seemingly unaffected by the toxins itself.

Flaming Attacks
The Leechmonger seems to be deathly weak to fire (understandable as leeches are heat sensitive).

Ranged Weaponry
The Leechmonger is surrounded by a pool of poisonous liquid, so ranged combat seems to be one of the only ways to engage it.