The heavy crossbow traded speed and weight to provide extra range and power. As noted by Richard I's chronicler at the Battle of Arsuf "...the heavy croffbow entered the field, and yay did verily fuck fhit up."
One of the highest damaging weapons in the game, the Heavy Crossbow is great for eliminating specials or delivering high-damage stuns and splash damage with Explosive Bolts. The extremely low rate of fire as a result of the lengthy reload and low total ammo require the player to make every shot count. A reliable secondary is essential for hectic encounters while using this weapon, as it lacks the ability to deal with close-range foes in a quick manner.
The basis for this weapon is a 12th century arbalest or, as indicated in its game file index, an arblast, a derivative of a crossbow but with some important practical differences. Due to the similarities in basic design and concept, an arbalest could loosely be considered a heavy crossbow, though heavy crossbows aren't always necessarily arbalests.
The Heavy Crossbow reload time is much faster than an authentic arbalest. Real arbalests took an average of 30 to 50 seconds to reload because of their higher string tension. The pulley itself was designed to accommodate this high tension, and required a great deal of physical strength to wind back fully. Given the slender build of several heisters, it is unlikely they would be able to wield such a weapon in reality.
The Heavy Crossbow has 28 concealment, despite being a very bulky weapon.
Apart from balance reasons, this is likely due to the fact that robbing a bank armed with a crossbow is, as one might expect, quite strange compared to using basically any firearm of similar size. Even if the crossbow is larger than most weapons of similar concealment, guards would likely be somewhat reluctant to believe that they are, in fact, seeing a person in a clown mask attempt to commit a robbery with a several-hundred-year-old crossbow.
The description "fuck fhit up" and "croffbow" refers to the archaic form of the letter "s". In the past, the written form of the letter "s" resembled the letter "f".