Sentinel

"The Sentinels run this place with an iron fist. They eat nothing but that fruit. They've never encountered Night Furies before, and that would be about the only dragon they haven't seen. And, speaking of seeing, they don't. But they seem to track pretty well for something that's blind."

- Hiccup

The Sentinel is a Boulder Class dragon that appears in Dragons: Race to the Edge, Season 5. Its name was first revealed via a clip on the Fandom article.

Official Description
"Although sightless, the Sentinels still keep close watch over the dragon graveyard on the Isle of Vanaheim. Often mistaken for stone statues, the Sentinels remain absolutely still when they allow ailing dragons to enter Vanaheim. But at first hint of an intruder, these sentries defend the sacred burial ground with sonic screeches and downdraft wingblasts. Born blind, sentinels have evolved their senses of hearing and smell to hyper-keen levels, making them incredibly alert. Yet despite their fearsome appearance, Sentinels show a a softer side by caring for the old and ill dragons who have come to spend their final days on Vanaheim. If you're unsure how to handle an unfamiliar type of dragon, just follow a Sentinel's lead. These observant dragons have kept vigil over every class and every breed to pass into their cemetery, making them the dragon experts of the dragon world."

- Dragonpedia

"Although sightless, the Sentinels still keep close watch over the dragon Graveyard on the Isle of Vanaheim. These vast, stony Dragons show little room for weakness, but a clever mind can exploit their blindness. Be wary, however; they have a keen sense of smell."

- Dragons: Rise of Berk

Egg
A Sentinel egg is round shaped and is covered in what appears to be rocky plate. The pattern and colour is reminiscent of an adult's back scales.

Hatchling to Adult
Sentinels are large dragons iconic for their tough rock-like epidermal layers that if pieced together with smaller stones, make them almost seemingly covered in moss. Their stature and bony wings resembles that of gargoyles. These scale clusters are more prominent on their faces and form rows of bony plates on their chest. They have a pair short horns on their head, complemented by similar ones on along their neck and one on their nose. These dragons are blind and thus have a translucent blue cataract over their eyes.

Titan Wing
Titan Wing Sentinels are almost identical to adults. However, instead of stony gray they are grey blue with black patterns. Their eyes are bright yellow.

Sonic Screeches
The Sentinels defend Vanaheim with their sonic screeches that can disorientate several dragons.

Downdraft Wingblasts
In order to chase dragons back to Vanaheim, the Sentinels often use their powerful wings to create a downdraft wingblast. These blasts are strong enough to knock dragons out off the air.

Remaining Motionless
The Sentinels are capable of remaining motionless for days at a time, which allows them to surprise and ambush unsuspecting adversaries or intruders in Vanaheim.

Teamwork
These dragons are known to work in teams. When stationary, they each have different posts and positions to take up around the island. Upon the threat of dragons leaving, they often corner them in the air in small groups.

Heightened Senses
Born blind, Sentinels have developed extremely sharp hearing and smell to navigate and track other dragons. They also possess the ability to remember scents after a long period of time and adapt accordingly.

Dragon Knowledge
Sentinels are extremely knowledgeable about the different dragon classes and breeds because numerous dying dragons go to Vanaheim. As such, they are said to know the strengths and weaknesses of nearly all known dragon species, except for a few, like the Night Fury. They use these to help care for and cater to the needs of different dragons, along with helping construct their funeral grounds.

Hibernating
As shown in Guardians of Vanaheim, the Sentinels hibernate. They will wake up eventually but it is a slow process. The only way to speed up the hibernation process is if the Sentinels are exposed to great amounts of heat.

Behavior and Personality
Sentinels are born solely to guard the funeral grounds of Vanaheim and assist sick or dying dragons there. As such, they rule the area with an iron fist as they forbid any dragons from leaving the area. They are also extremely aggressive to the Grim Gnashers, which are perfectly healthy and prey on sick dragons.

They guard the area on various posts around and on the island, perpetually stiff and stationary like statues. However, upon the slightest sign of threat, they crackle awake.

However, they do exhibit kindness and fondness to the inhabitants of Vanaheim. They help to construct funeral grounds and provide them with food. The former action makes Fishlegs believe that these dragons are capable of abstract thinking and knowledge of life and death.

Blindness
As aforementioned, Sentinels are born blind. Despite having sharp hearing and smell, they can be outsmarted by several dragons that cover up their scent with Vanaheim Fruit and move silently.

Unknown Dragons
Dragons that the Sentinels don’t know too well like the Night Fury and any other dragon that never rested on the island can easily avoid them, as proven by Toothless.

Training
While no Sentinels have been formally trained or bonded by anyone, by showcasing the right intentions, they appear to be less hostile. By helping them guard the area and proving that you are of no threat, Sentinels will grant you the respect and lower their aggression, even allowing you in and out of the island.

Dragons: Race to the Edge, Season 5
These dragons made their debut in the episode "A Matter of Perspective" when the dragon riders go to Vanaheim. They also appear in the episode "Searching for Oswald... and Chicken".

Dragons: Race to the Edge, Season 6
The Sentinels return in the episode "Guardians of Vanaheim".

Dragons: Rise of Berk
The Sentinel was introduced in this game along with its Titan Wing form and a premium individual, Silent Knight.

Trivia

 * The Sentinel shares statue-like similarities to the Victorian-era gargoyles.
 * Even though the Sentinels are blind, they have small pupils.
 * The word "sentinel" means "guardian" which is what Sentinel dragons are for Vanaheim.
 * The Sentinels’ protective behavior towards Vanaheim is possibly inspired by that of the Dragon Guardians of Tomorrow.