Librarian

Librarian's are hulking beasts named for their habitat, the Moscow state library which is the only part of the game in which they are encountered.  Despite their heavy frame librarians are incredibly agile, capable of moving faster than Artyom even while sprinting and have immense physical endurance, great enough to withstand a direct hit from a sticky grenade. While a powerful foe, Artyom does have an advantage in that they will not attack the player as long as eye contact is maintained, however this tactic is often unreliable within the game. 

 
Darker, more aggressive variants of these creatures can be found in the lower levels of the depository. However these variants are normally sleeping and as such can be avoided by stealthy players. They stalk the player throughout the depository level, and often appear suddenly instigating a jump scare, for example smashing through a closed door or appearing and grabbing Artyom through a hole in the wall. There is also an achievement  (named "heavy reader") unlocked by killing  a Librarian.