Sentinels are weapons of mass destruction created by the Elders to inhibit the growth of sentient life in the Milky Way galaxy. The Sentinels had been inactive for millions of years prior to Elder weapons being used in the war between the Rindhalu and Maxolhx, at which point, they emerged and took both species to the brink of destruction. The technological disparity between the Elders and the current species of the Milky Way renders the Sentinels virtually unstoppable, and therefore, fear of reawakening the Sentinels has resulted in a long-term stand-off between the Tier 1 species, with the use of Elder weapons seen as mutually-assured destruction.
Sentinels exist in Higher Spacetime, only emerging into normal space to carry out an attack. Once in normal spacetime, they are massive, with their bodies spanning several kilometres. According to Joe Bishop during his encounter with the dead sentinel fragment at the Roach Motel, they appear to look like large spiders with many jagged protrusions sticking out of their long appendages. However, Skippy states that this is not how they truly look, but instead that this one only appears the way it does because it is currently residing in our space-time.
The Sentinels are effectively impervious to kinetic and directed energy weapons. The true range of their offensive weapons is unclear, but they are known to possess the ability to manipulate spacetime, cause solar flares, crush objects into tight spaces, absorb objects into themselves, knock planets out of orbit, and prevent starships from making jumps.
Fragments of Sentinels in local spacetime are just lumps of inert elements and the Elder technology it is composed of cannot be reverse-engineered.
SPOILER ALERT! The section(s) below SCHMAYBE includes Spoilers.
The Sentinels were created by the Elders as part of their three-part solution to prevent sentient species from evolving in the Milky Way to the point where they have the ability to disrupt the Galactic Shield. The Sentinels were the weapons through which sufficiently evolved species were destroyed. The Sentinels can not pick their own targets; they are directed to a target by a Master Control AI.
For millions of years, the Sentinels, directed by AIs, destroyed civilisations that advanced beyond a threshold technology level. This likely included knocking the planet Newark off its regular orbit, crashing its climate and destroying the native civilisation.
Following The AI War, the destruction or incapacitation of the Master Control AIs meant that the Sentinels no longer received instructions. Over time, they went into hibernation, allowing spacefaring species to emerge in the Milky Way. The Sentinels briefly emerged during an early phase of the Rindhalu / Maxolhx war, causing massive destruction to both species before returning to hibernation. The prevailing opinion by the Senior species is that the use of Elder weapons awakened the Sentinels, though Skippy is unclear why this would have occurred, and how the Sentinels would have targeted planets of the two species without direction from a Master Control AI.
The Flying Dutchman encountered a Sentinel in the Waterloo system. Although it never emerged into local space, Skippy was able to detect its presence. The Sentinel manipulated spacetime in order to prevent the Dutchman from jumping out of the system, trapping them there for weeks. It was believed to be the only awakened Sentinel in the galaxy at the time, although why it was not in hibernation was not known. The Merry Band of Pirates encountered a "dead" Sentinel in the Roach Motel. The investigation of its wreckage inadvertently led to the infection of the Flying Dutchman with an Energy Virus.
Sentinels began to emerge across the Galaxy following the Humans resetting the Wormhole Network in an attempt to trick the senior species into believing they had the capacity to crash the network at will. Sentinels began to destroy planets and mining operations throughout the galaxy, seemingly at random. The senior species blamed the Humans for waking up the Sentinels through their misuse of the Wormholes. Skippy speculated that the Sentinels had actually been awakened through a broadcast by another Master Control AI, noting the Sentinels appeared confused and lacking direction, due to them not receiving targeting from an AI.
Skippy was able to destroy a Sentinel by fooling it into absorbing a missile containing the same energy virus that had destroyed the Sentinel at the Roach Motel. He later speculated that the virus was intentionally designed for that purpose, held by the Guardians as a weapon to prevent rogue Sentinels from disrupting the Galactic Sheild. With support from a Maxolhx fleet, Skippy was able to get close enough to the Sentinel at Waterloo to broadcast a message to put the remaining Sentinels back into hibernation, but shortly after, Valkyrie was captured by the Sentinel and dragged into higher spacetime.
When a joint Rindalhu / Maxholt fleet threatened Earth, Valkyrie reappeared with a Sentinel (nicknamed Roscoe), which appeared to be under Skippy's control.
