Eagle Vision

"Why do we have these gifts? These abilities? Because it's in our blood!"

- Subject 16 about eagle vision.

Eagle Vision was a sixth sense that certain lineages of Assassins possessed, and was known to be only accessible to the descendants of humans who interbred with Those Who Came Before. Those who possessed this gift were able to instinctively sense how people and objects related to them, which manifested as a colored glow, much like an aura in a person's mind's eye.

When one learned to fully use eagle vision, the ability evolved to the more advanced Eagle Sense. This heightened all of the senses of its user, allowing them to eavesdrop on conversations, detect the heartbeat of a target in the area, or even foresee a target's path; which assisted in planning ambushes or setting traps.

Ezio Auditore
"Listen closely: Return to the house. In my office is a hidden door. Use your talent to find it."

- Giovanni Auditore instructing Ezio to use his eagle vision.

Ezio Auditore, much like his ancestor Altaïr Ibn-La'Ahad, possessed eagle vision, and made use of it to locate hidden passages, as well as to predict the intentions of his allies and enemies. Additionally, he was also able to see the map hidden beneath the pages of the Codex with it.

Though never referred to explicitly as "eagle vision," Giovanni Auditore once referred to Ezio's ability as his "talent," and Salaì spoke of it as his "gift."

Others
"The note told me to use my other eyes. Sometimes I see things that are not there. Drawings, or waves."

- Giovanni Borgia describing his eagle vision.

La Volpe was thought to possess an advanced form of eagle vision, as it was claimed he could "look through walls."

Giovanni Borgia also had eagle vision, and was able to use it to locate and tail Francesco Vecellio through a crowd. With it, he was also able to see things that were not really there. Francesco called the ability "using his other eyes."

Assassins
By using the Animus to follow the memories of his ancestors, Desmond Miles also gained the ability to use eagle vision, likely as a result of the bleeding effect. Through it, he was able to see Lucy Stillman (a fellow Assassin) glowing blue and Warren Vidic (a Templar) glowing red.

He was also able to use this ability to see the cryptic messages that Subject 16 had written in the Abstergo laboratory with his own blood.

During his escape from Abstergo Industries, Desmond discovered another application for his newly-acquired power. Much like thermal vision, he used it to discover the code of a keypad, by seeing the heat signature or fingerprints left upon the correct numbers.

In a video clip hidden within one of his Glyphs (which needed to be scanned with eagle vision to be unlocked), Subject 16 stated "Behold the Assassins, the children of two worlds! Why do we have these gifts? Because it's in our blood." This implies that, because Altaïr, Ezio, Desmond and Subject 16 all have eagle vision, their bloodline is connected to Those Who Came Before.

Later, upon escaping to Monteriggioni, Desmond was also able to use eagle vision to see a message Ezio had left upon the wall of the Sanctuary stairwell, which bore the password to the Colosseum Vault.

Templars
Abstergo trainees using Animi to hone their skills were also granted an ability called Templar Vision. This rendered ability mimicked eagle vision, and allowed the user approximately 3.5 seconds of the gift.

Assassin's Creed

 * An Assassin guard in the Kingdom can be seen as glowing red, but reacts as all other guards do. This guard was originally supposed to have been Jamal, an Assassin who aided Masun in betraying the Brotherhood; however, the sequence where Altaïr assassinates him was cut from the final game, leaving him as a minor Easter egg.
 * In the original Assassin's Creed, movement was prohibited while using eagle vision, and the player was required to be at maximum synchronization. This was changed, however, in succeeding games.
 * Assassin's Creed states that the eagle vision ability is actually just the Animus' visualization of the Assassin's observational skills. However, as Juno later refers to Assassins seeing "the blue shimmer," the backstory for eagle vision was likely changed to be a real-world ability.

Assassin's Creed II

 * When using eagle vision, Ezio himself is marked with blue, indicating him as an ally to any others with the ability.
 * In the beginning of the game while playing as Desmond in the Hideout, use Eagle Vision and you will see that some equipment will shine blue as if they are allies. They are marked blue to show Desmond the objects that can be used for free-running.
 * Lucy makes a reference to the ability by saying "Just open your eyes" when Desmond asks her how he would be able to find the Hideout's defense system sensors, which she had tasked him to switch on.

Assassin's Creed: Brotherhood

 * When traveling through Rome, eagle vision can be used to mark Flags, feathers or treasure chests on the map.
 * While using eagle vision and roaming Monteriggioni as Desmond, a red trail is visible leading from the fountain with the Assassin insignia, until the door to Mario Auditore's study. The trail is colored similarly to the blood glyphs of Subject 16, and seems to be made up of many footprints that form a double helix (or DNA strand). Or the bleeding caused by Ezio when he was shoot by an arquebusier.
 * When outside of the Animus, there is sometimes a pause that causes allies to glow gold for a few seconds.
 * Unlike in Assassins Creed II, enemies and allies do not glow their standard colors after they die, and will just appear grey like civilians.
 * When eagle vision is activated in some memories, water is shown to be glowing white like hiding spots.
 * Unlike eagle vision, Templar vision marks targets with blue, rather than with gold because it is Templar hunting Templar and they are allies.

Others

 * A mini-game in the Assassin's Creed room of PlayStation Home features an eagle vision puzzle that, when completed, opens up the Hideout.