The Lightning Arc, like every god-damned forsaken weapon with the word 'lightning' in its name, is actually not a lightning weapon. So that means it has no relations with Tesla Weapons nor Arc Weaponry.
Rather, it is a weapon that stores solar energy before it is released as a wave of living energy tendrils which envelop targets, probing for any points of weakness. Yes, I know, that sounds dangerously close to a C'Tan, but it functions similarly to an AEonic Orb insofar as both weapons using the energy of a star. However, the way it functions is just...weird.
Fortunately, in terms of actual gameplay, in Battlefleet Gothic: Armada II, the Lightning Arc is one of the primary weapons used by Necron starships. It is by far the most common of these naval weapons and it is a rough analogue of a Macrocannon Battery. Being the Necron's run-of-the-mill starship weapon, Lightning Arcs are found on either side of the ships' hull.
Like all Necron starship weapons, Lightning Arcs essentially bypass all forms of shields including Void shields to get into the soft and juicy bits. However, because of its function, it affects the entirety of the ship as the quasi-living solar energy attacks weak points of the target. This makes the Lightning Arc, like its namesake, quite good at creating attack-chains from one targeted ship to the next nearest one.