A Narvhal is a Tyranid Bio-ship that fulfills the essential role of allowing all the vessels of a Tyranid Hive Fleet to travel faster than light. This ability makes up for a Narvhal ship being small, innocuous, and having almost no weaponry or armour.
Tyranids do not travel via Warp jumps, such as the Imperium does, and so instead rely on their Narvhal ships. Through the use of monofilament spines clustered along a Narvhal's bow which can interpret a wide range of sensory input and even a broad spectrum of gravimetric signals, a Narvhal can detect the presence of a planetary system at an incredible distance away. It can then somehow harness the system's gravity to create a corridor of compressed-space through which Tyranid vessels can travel towards the system at a swift rate. Whilst slower than proper warp travel, this method is infinitely more reliable.
However, strong gravitational forces cause interference with a Narvhal's delicate senses. And so on final approach to a system, a Tyranid fleet can no longer use a Narvhal's ability but must resort to slower conventional propulsion. This will often slow their arrival by years or even decades.While within system boarders, they must resort to conventional propulsion methods.
Disastrous side effects are caused by this method of manipulating a system's gravity. The ensnared planets will often experience earthquakes, solar flares, tidal waves or other natural disasters in the period between a Narvhal harnessing the system's gravity and the Tyranid fleet actually arriving. For the Tyranids this is of course another weapon as the defenders will be busy and disorganised dealing with their own planet turning against them.