Dropped by Green Slimes, this item is the basic building block of the next tier above the default equipment. Weapons can be upgraded using Green Slime by anyone, but armor can only be crafted by those of the Guardian class. The reason why it can only be crafted by the guardian class is their excelling ability with armor and defense-type status. Because they have this ability, they can craft armor from these raw materials.
A main use of the slime core is the ability to attach it onto a weapon to deal blunt damage, which allows regular damage to be dealt to green slimes, and it's very important to surviving attacks by slimes. More slime cores can potentially add extra damage capabilities to the weapon of choice. However, a max of three slime cores can be added onto a weapon before the damage will not be able to raise, and not all weapons can be impacted by simple slime cores.
In terms of the benefit of armor enhancements by slime cores, a damage debuff of 5 HP retained from an attack by most monsters that do not have pierce capabilities, which will null any upgrade to the armor equipped and will only perform damage based on the [b]base[/b] stats of the armor equipped. Monsters with attacks like this include the whole dragon family, and the berserker orcs. In the early game, this sort of upgrade is incredibly useful.
Other uses of the slime core have yet to be discovered, however, as it's still early in the open beta, where many new features had been added and have yet to be identified.