Eagle Vision

Eagle Vision is a supernatural ability that Ezio Auditore, Altaïr Ibn-La'Ahad, Desmond Miles, Subject 16 and presumably others of Assassins of the same lineage possess. 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 persons "mind's eye"). This ability is known to only be accessable to the decendants 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.

Eagle Vision

 * Red = Enemies
 * Blue = Allies
 * White = Sources of information/hiding places
 * Gold = Targets

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

Trivia

 * 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, but the sequence where Altaïr assassinates him was cut from the final game, leaving him as a minor easter-egg.
 * 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 the original Assassin's Creed game, movement was prohibited while using Eagle Vision, and the player was required to be at maximum synchronization. This was changed in Assassin's Creed II so that the player can use Eagle Vision even while moving and without maximum synchronization.