Overview
Delta Green is a standalone tabletop roleplaying game of modern conspiracy and cosmic horror. Originally a supplement for Call of Cthulhu, the current standalone edition was published by Arc Dream Publishing in 2016. Players take on the roles of federal agents or contractors working for a secret, illegal organization dedicated to containing supernatural threats and covering up the existence of the Cthulhu Mythos to protect the United States.
The system it directly evolved from is
Description
In Delta Green, the horror is not just about monsters; it is about the toll that a life of secrecy and violence takes on the human soul. Agents juggle their high stakes investigations with their crumbling personal lives. They must navigate a world of bureaucratic red tape, government conspiracies, and sanity shattering entities while ensuring that no one, not even their own families, ever finds out the truth. The game is famous for its bleak tone and focus on the cost of the "Great Work."
System Overview & Key Features
d100 Percentile System
The game uses a familiar percentile dice engine. Skills are rated from 01 to 99. To succeed, you must roll equal to or under your skill rating. If the tens and ones digits match (a double) and the roll is a success, it is a critical success; if it is a failure, it is a critical failure.
Bonds and Domestic Ruin
Agents have Bonds representing their relationships with loved ones or colleagues. When an agent suffers Sanity loss, they can choose to project that trauma onto a Bond to reduce the loss. This mechanically represents the agent coming home and taking their trauma out on their family, slowly destroying their personal life to stay functional in the field.
Sanity and Adaptation
The game tracks three types of Sanity loss: Violence, Helplessness, and the Unnatural. If an agent experiences enough Violence or Helplessness without breaking, they can become "adapted," making them more resilient to those triggers but less capable of maintaining human empathy and Bonds.
Lethality Rating
Instead of rolling many dice for heavy weapons like assault rifles or explosives, Delta Green uses a Lethality percentage. If the player rolls under this rating, the target is instantly killed. If they roll over it, the weapon deals a fixed, significant amount of damage instead.
The Home Phase
Between missions (Operas), players play through a "Home" scene. They must choose how to spend their limited time: fulfilling responsibilities to maintain Bonds, seeking therapy to recover Sanity, or practicing skills. Choosing one often means neglecting the others, leading to a downward spiral of isolation.
Special Training and Agencies
Character creation is deeply rooted in real world professions. Whether you are a CIA Case Officer, an FBI Special Agent, or a CDC Epidemiologist, your profession dictates your starting skills and the federal resources you can leverage during an investigation.
Additional links
deltagreen.com 
https://deltagreen.com - Official Delta Green website
arcdream.com https://arcdream.com - Official Arc Dream Publishing website