The Sword Core empowers Ronin's melee and sword abilities, and gives access to new sword attacks. While active, the Leadwall will be replaced with an empowered sword, which does heavy damage and has infinite uses while the core is active. Also, abilities like Sword Block and Arc Wave will also be amped, and will have boosts that recharge insanely fast.
It can be extended using the Highlander Kit, which will allow you to recharge your core anytime you get a Titan Kill. Using this in Frontier Defense will allow you to have a core for almost the whole round, as Titan kills are relatively easy to get.