Eagle Vision

Eagle Vision is a supernatural ability that certain lineages of Assassins possess, like those of Marcus Junius Brutus and Altaïr Ibn-La'Ahad. The user is able to instinctively sense how people and objects relate to him, which manifests as a colored glowing (much like an aura would in a person's "mind's eye"). This ability is known to only be accessible to the descendants and bloodlines of the humans that paired with Those Who Came Before.

Assassin's Creed
To use Eagle Vision, the player must press the Head button. Depending on control settings, the Head button will most likely be either (for PlayStation 3), Y (for Xbox 360), or E (for PC default layout). In addition, the Synchronization bar must be at its current maximum capacity, otherwise pressing the Head button will instead result in a first-person view with no extra benefits.

At the end of Assassin's Creed, Desmond gains the ability to use Eagle Vision outside of the Animus. He sees Lucy Stillman (a fellow Assassin) glowing blue and Warren Vidic (a Templar) glowing red. Using Eagle Vision will also reveal cryptic messages Subject 16 wrote with his own blood in the Abstergo laboratory.

Assassin's Creed II
During his escape from Abstergo, Desmond discovers a new application of this power; using it on a keypad which Lucy lacks a code to unlock reveals the fingerprints of previous users. A simple process of elimination thus enables him to unlock the door.

Ezio, like Altaïr, was able to use Eagle Vision. His father and uncle also possessed this ability, and it is thought that La Volpe also possessed an advanced form of this skill as well, as it is claimed he can "look through walls". It is used to detect and scan Glyphs from Desmond's perspective.

In a glyph narration from Subject 16, he says, "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. This theory can be supported by the fact that both Subject 16's blood and light patterns in the vault glow when seen with Eagle Vision.

Also Eagle Vision can now be used in 3rd Person and while Ezio is moving, allowing it to be a lot more useful during missions.

Assassin's Creed: Brotherhood
When roaming Rome with Eagle Vision on, whenever you spot a feather or flag, it will be marked on your map, so you can pick it up whenever you want later.

Subject 16 has also hidden objects similar to Glyphs in Assassin's Creed II in Rome, called Rifts. These Rifts are small openings in structures that expel rays of light continuously. Unlike the Glyphs, the Rifts can only be seen while in Eagle vision, as turning it off will render the rift invisible, and otherwise inaccessible.

In the multiplayer mode a similar ability, Templar Vision, exists as an ability. The ability gives you a 3.5 seconds of eagle vision. Targets are blue while pursuers are red.

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 a visualization of the Assassin's observational skills, created by the Animus. 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

 * Giovanni Auditore tells Ezio to "use his talent" to find a hidden door in his office, showing that Giovanni was aware of his son's ability.
 * When using Eagle Vision, Ezio himself is marked with blue, indicating him as an ally to any others with the ability.

Assassin's Creed: Brotherhood
In Assassin's Creed: Brotherhood dead enemies and allies don't glow their normal colors unlike in Assassins Creed II, they will just appear grey like civilains.
 * Whilst using Eagle Vision and roaming Monteriggioni as Desmond Miles, 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).
 * In Assassins Creed: Brotherhood, when outside of the Animus, there is sometimes a pause that cause allies to be gold for a few seconds.

Others

 * A mini-game in the Assassin's Creed room of PlayStation Home features an Eagle Vision puzzle, which, when completed, opens up the Hideout.
 * In Assassin's Creed: Project Legacy, Giovanni Borgia also displays this ability, and uses it to locate Francesco Vecellio. Francesco referred to it as using his "other eyes."