Ravnica

This collection of decks is based on Ravnica, which a plane of existence in Magic: the Gathering, by Wizards of the Coast. Ravnica is a vast city ruled by ten guilds, each of which represents a particular combination of two of the five colors in Magic. The personality of each guild is designed to reflect the combination of the philosophies of those two colors. In Sentinels-land, I've translated that into one environment deck for each guild.

Environments

Azorius Senate

The Azorius Senate governs the laws and regulations of Ravnica. They are extremely bureaucratic and are mainly known for impeding criminals - and law-abiding citizens. Mechanically, the environment tends to punish heroes and villains "fairly" for taking action, like dealing damage whenever damage is dealt or cards are played. It also reduces or prevents damage. Generally, a game with Azorius should take a little longer to finish and occasionally require some difficult decisions about whether the right option is to do nothing, but not to an extreme degree (compared to, say, two healing vats from Pike Industrial Complex).

Boros Legion

The Boros Legion enforce the laws and regulations of Ravnica - forcefully. They have a militaristic code and can muster legions of angels, foot soldiers, and skyknights in battle. Mechanically, the environment is primarily composed of soldiers that protect and support each other while hitting non-soldiers. It's is fairly consistently a net negative for the heroes, but as long as they don't let it scale out of control, it should be manageable.

Cult of Rakdos

The Cult of Rakdos is a sadistic and hedonistic death cult of pleasure-seeking entertainers. In general, they're super murder-y, but most of their victims are willing. As an environment, it has a variety of mostly symmetric punishments and a tendency to pick on low HP targets. It's potentially dangerous for the heroes, but with the right setup and timing it can actually be close to a net benefit thanks to its tendency to punish the villain as well.

Golgari Swarm

The Golgari Swarm are necromancers that worship the natural cycle of life and death. They see life and death as being equally natural - and with their magic, the distinction between the two is quite fluid. As an environment, they manipulate trash piles - filling them by discarding from the top of decks, using them as fuel for their effects, and returning targets from trash to play. This generally increases the power of all target-heavy decks, though several of the cards have ambiguous effects designed to give the players the ability to influence the effect the environment has on the game.

Gruul Clans

The Gruul Clans are a chaotic and decentralized collection of wild warriors and monstrous creatures. They were responsible for protecting the wild places of Ravnica, but they mostly failed, and the other nine guilds converted essentially the entire plane of Ravnica into a massive cityscape. This has made the Gruul very angry, so they mostly just smash things now. This environment has a lot of high HP targets and indiscriminate smashing, with a subtheme of punishing non-one-shot card plays (which are very loosely analagous to noncreature spells, which the Gruul punish in Magic).

House Dimir

House Dimir spies on, influences, and sabotages the other guilds of Ravnica from the shadows. For centuries, they were believed to have never existed, though they recently emerged to take their rightful place as the tenth guild. Their agents have infiltrated every other guild to varying degrees, and they have a deep and subtle influence on the direction of the plane's activities. As an environment, the Dimir have a variety of spy targets that have special effects when they deal damage to players. These effects mostly involve discarding cards from decks, which cause various other effects to trigger. The environment doesn't generally deal massive damage or have as large of an impact as some others, but its presence is noticeable, and keeping the spies to a minimum is generally wise.

Izzet League

The Izzet League is basically a whole guild of mad scientists and their crazy experiments. They love casting spells and examining their results, which in Sentinels translates to one-shot cards. Sometimes the deck rewards one-shots, sometimes it punishes one-shots, and sometimes it makes you play free one-shots. Sometimes it also just does some weird stuff because hey, science-magic! In general, this environment should have a significant effect on the game, but shouldn't make the game that much harder.

Orzhov Syndicate

The Orzhov Syndicate runs the banks of Ravnica. Over the course of the game, you'll inevitably rack up Debt - either in exchange for rewards from the environment, or as a punishment for destroying detrimental environment cards. Debt always comes due eventually, and the environment has Penance cards to extract a price from debtors. Generally, this environment is a slow burn, and it's not too dangerous - especially if you can manipulate the villain into taking on debt. However, it makes its presence felt with some narratively resonant extra decision points.

Selesnya Conclave

The Selesnya Conclave is a collective of spiritually inclined nature-worshipping hippies. They love life in all its forms, but especially the smaller forms that are cruelly ignored by the rest of society, and work aggressively to encourage the spread of life throughout the world. As an environment, it generally protects low-HP targets and punishes characters. If you're in a target-light game, it can be helpful for the heroes, but villains with a lot of targets will enjoy its protection and buffs.

Simic Combine

The Simic Combine is a collection of increasingly mutated wizards who work to maintain the natural world - mainly by mutating it wildly so it's too strong for anyone to stop it. If the Izzet are mad scientists, the Simic are mad biologists, creating ever more absurd and dangerous mutations and spliced life-forms. The environment has a number of targets and a token theme where each token makes the targets stronger. They also have a few experiments to mix things up. This environment is consistently dangerous to everything that isn't a mutant, and is prone to mutating out of control if left alone for too long.