Monoliths are peculiar claw-shaped alien structures that sprout from the ground, usually in support of an enemy ambush or defensive effort. They are first encountered by Darius Mason while investigating a distress signal from a mining colony.
The Organismic enhances all creatures within a wide radius, making them tougher to kill. Creatures enhanced by a monolith have a pulsing green barrier around them, serving as a type of armor to protect them.
The monoliths attack by charging up a large ball of greenish plasma energy, which explode upon contact, inflicting heavy massive damage. Which need to be avoided immediately. 
Surprisingly, The Maul does the most damage to a Monolith. However, be careful after killing it, so better get away as it will explode after being killed as it can instantly kill you if it occurred especially in the highest difficulties.