Sentinel (No Man's Sky)

The Sentinels or Aerons are Mechanical Lifeforms; self-replicating, non-organic machines that are similar to the  Korvax, another Mechanical Lifeform in No Man's Sky's Universe. Their origins are unknown but they are seemingly deployed on every known planet to balance the efforts and actions of the player within the procedurally generated ecosystem. Sentinels act as the workhorse machines of  The Atlas and as universal police, policing the actions of those who reside within the universe. They are, in essence, the "gardeners" of the No Man's Sky. If the player makes an attempt to alter the environment while they are around, they will not hesitate to defend their "garden." They will attack the player if caught harming the ecosystem of a Planet. A Sentinel will also defend itself if the player shoots it, alerting other Sentinels in the area.

Drone sentinel
Drone sentinels are the most common and best known type of sentinel They are active on basically every planet in the universe. When a sentinel drone is killed it can drop: 1-50 zinc, 1 neutrino module, 10-45 pugneum, blueprints.

They are armed with 2 plasma cannons. Most of the time they are not a threat to an average player.

Upon gaining a wanted level of 2 or higher, the player will encounter an armoured variant of the drone which is considerably more resilient than the standard model.

Quadrupedal sentinel
Quadrupedal sentinels also known as dog sentinels are the second type of sentinel that appear (they appear mostly at wanted level 3). When a quadrupedal sentinel is killed it can drop: 70-120 titanium, blueprints, dimensional matrix. They are armed with one phaser cannon, and can be a threat even to experienced players.

Bipedal sentinel
Bipedal sentinels also known as sentinel walkers are the last and most powerful type of planet based sentinels. They appear when the player reaches wanted level 4. When a bipedal sentinel is killed it can drop: 70-120 titanium, blueprints, dimensional matrix.

They are armed with 1 phaser cannon and possibly 1-2 plasma cannons. They are a threat even to the most experienced gamers.

Sentinel spaceship
Sentinel spaceships, also known as sentinel starfighters or Sapporoz S9 (the name they have in the game) or sentinel interceptors are the first sentinel type that appears in space. They appear when a player reaches wanted level 1 in space. When killed they can drop: titanium, 20-100 tropheum, 1-4 dimensional matrix, 1-8 neutrino modules.

They are armed with 2 phaser cannons and because of their speed and maneuverability they are difficult to beat.

Sentinel cruiser
Sentinel cruisers also known as sentinel freighters or sentinel mothership are the largest and most powerful type of sentinel and the closest to a boss you got in no man's sky. Unlike all the previous types there can always only be 1 single of these in a system. They can't be killed, the only thing that you can do is destroy all their weapons. They appear when a player reaches wanted level 5 in space.

They are armed with 2 defence turrets which shoot plasma bolts and 6 phaser cannons on each side, they are always a threat and even the best players have a hard time beating this ship.

Motivations
According to an article published in the January 2015 Game Informer, it is suggested that Sentinels, formerly known as The Malevolent Force, were not all that malevolent to start off with:

"This robotic enemy army isn't all that malevolent as far as motivations go. They are simply trying to maintain the balance of the universe. On some planets they patrol, keeping an eye out for anyone who kills creatures, carves out resources, or generally changes the environment. If you're just strolling around scanning things in, they leave you alone. If you draw their attention, they attack like traditional first-person shooter enemies, shooting lasers, strafing, and taking cover."[1]In a separate interview with The Guardian published in October 2014, Sean Murray said:

"It's dangerous on planets. Each one has an ecology, and you're not always at the top of the food chain; we've shown large creatures that attack you. There's also something else we haven't shown. There is a malevolent force in the universe that does provide a lot of combat and more core gameplay at times. We wanted that; we wanted players to wander around exploring planets but never to feel entirely safe."[2]

Background information has been found within abandoned structures on 'Derelict' consoles. These belong to a race that was active before the Vy'Keen, but were annihilated during a war with the sentinels. This information describes the inability of any race to ever truly destroy enough of the Sentinels to eliminate them, and alludes to the mystery of how they are produced.

Lore
According to records scavenged from various ruins, traveler logs, and plaques - Sentinels are also known as The Aerons, and have existed longer than most of the entities. The exact time of their first appearance in the Outer Edge is yet to be discovered. They quickly spread across the entire No Man's Sky universe due to the fact that the Vy'keen Alliance were attacked by the Gek Dominion coming from the core while they desperately tried to remove the Aerons from existence.

When the wars ended, the Aerons enslaved Gek the First Spawns to work for them and the Geks quickly come to realize that these sentinels were far more vicious than themselves. They are ultimately so powerful that even the more intellectual entities such as the  Korvax started to worship them and deemed the Aerons to be the rule-keepers among the galaxies.