It was jet black. A shade of black so deep your eye just kinda slides off it. And it shimmered when you looked at it. A spider big as death and twice as ugly. And when it flies past, it's like you hear a scream in your mind.—Lt. Warren Keffer, The Fall of Night
These ships make up the bulk of the Shadows' fleet. These ships are alive, but cannot operate independently, instead they require a humanoid to be merged with them and operate as its CPU. The unfortunate "pilots" are usually captured members of one of the younger races. Before being merged a person will have bio-mechanical implants inserted into his or her brain. First the implants facilitate the person's link with the ship, then they suppress the subject's free will, making him or her a slave to the "machine". Once the process is finished, the person is a mindless servant of the Shadows who will obey their every command. The reason the Shadows have chosen not to give their most powerful servants free will dates back to the time when the technomage order rebelled against their Shadow creators and chose to follow their own path rather than serve as cannon fodder for the Shadows. If a pilot is not properly trained or has the implants removed before entering the ship it is speculated they go instantly mad and the ship acts in such fashions, as seen on the unearthing of the Ganymede battlecrab.
Battlecrabs are extremely powerful ships. The primary weapon is a violet energy beam of variable output. It can target precisely enough to cut a warship in two without causing secondary explosions, but it is also suitable for large scale orbital bombardment (seen when a task force of three Battlecrabs destroy the Narn's largest civilian colony in just a few shots). This beam also functions as point defense weaponry against enemy fighters. The skin of a Battlecrab is capable of absorbing energy weapons and can kill a human (and probably other species as well) through physical contact. They also have a jump point disruption weapon that will collapse a jump point, usually destroying the ship entering or leaving it in the process. The Shadows' ships do not use jump points to travel between hyperspace and normal space (as all the younger races and the Vorlons do), but instead phase in and out of hyperspace, giving the appearance of a cloaking technology. This testifies of the Shadows' unparalleled understanding of hyperspace. The ship can also remain somewhere "in between" hyperspace and realspace for an unspecified amount of time, making it invulnerable (but also unable to attack) in both dimensions. This would seem to be the perfect cloaking device. The Shadow battlecrabs are powered by an unknown type of generator system that has been conjectured to tap into the tidal forces of hyperspace, giving the ship all the power it may need for a virtually infinitely long time (where vessels of the younger races generally have to resupply, take on more reactor fuel, etc.). They carry a contingent of at least 40 fighters which they can launch in the form of a sphere into groups of opposing vessels before they disperse and attack. Though powerful, they are not indestructible, and a weakness of their organic nature is that they and/or their living CPUs feel pain. Attacks that exceed their pain threshold can cause them to become momentarily paralyzed, which makes them somewhat vulnerable to focused, continuous fire—provided the attackers are not destroyed by the pained vessel's allies. In addition, when three Narn Starcruisers manage to destroy one of the "spines" on an attacking Battlecrab in The Long, Twilight Struggle, the vessel is incapacitated for the remainder of the battle (despite the relatively minor damage) and has to be towed away by another Battlecrab afterwards.
Battlecrabs also have the ability to 'merge' creating larger and more powerful capital ships. This is also the way the Shadow ships help their "injured" comrades. Also, the battlecrabs seem to have an "attitude", and one of a pitbull dog at that: Delenn has said to Sheridan that "once these ships target you, they will not tire, will not stop, until they have destroyed you".
The size of the battlecrab is a difficult thing to determine, with the ship's design being so alien, with no apparent distinguishable features. However, it has been stated these ships are alive in a way, and are not so much constructed as "grown" somehow. Thus, based on visual evidence from the show, the battlecrabs appear to come in various sizes, answering the need for medium to very large capital ships with just one basic model. The smallest seem to be about a kilometer long, counting in the spines, the most common ones are about 1 mile (Script error km) in length and the largest ones seen look about 3,000 metres (Script error mi) long. This is not to say that the Shadows are not in possession of, or at least capable of creating, these ships in a wider size range.
Because the battlecrabs contain a living host, they are vulnerable to telepathic attacks. In an attempt to counter this, the Shadows kidnapped a number of human telepaths and attempted to use them as CPUs in their ships to counter enemy telepaths.
In the expanded universe produced by the RPG books from Agents of Gaming and Mongoose Publishing, it is known that Shadow Battlecrabs are living organisms and actually grow from smaller versions of the class. It is also known that eons ago during the height of the First Ones' empires, the Shadows may have actually piloted their own craft but gave up the practice.
Earth's involvement with the Shadows began when a dormant 'battlecrab' was found on Mars. This relates to the Shadows' repeated practice whenever they were about to lose one of their numerous campaigns to hide away their capital ships by burying them deep underground "on a thousand worlds" until such time when the Shadows will be on the rise again. Then, they will start "calling" their ships, which would come, usually all by themselves, back to Z'ha'dum, the planet the Shadows have long ago adopted as their home.