Hekatonkheires is a useful magic that is a Caster Magic.
Mages of the Hekatonkheires magic often will have distortions appear around them beside their arms. And some of them have even given themselves a pair of extra arms on their flesh. But, when a Hekatonkheires Mage uses a spell, a set number of arms appear out of the distortions ready to attack or grab or block. This magic often gives the user the ability to do multiple things at once. All their arms are within reasonable distance of the body. Meaning they can't have an arm appear fifty feet away from them grabbing someone. They all have to be in reasonable distance of the body, their personal bubble if you will.
Some mages of this magic will carry various tools and weapons with them to wields after casting this spell. One mage is even rumored to be called the Sword Army Mage. A mage that successfully summoned 100 arms with short swords to fight against another mage having the amount of arms an army would have and succeeded in the battle. The more arms that a mage summons the more magic power they have to use up. And maintaining the arms isn't easy either, only a hand full of Hekatonkheires Mages have been able to hold up a full cailber spell (100 arms) for more than an hour. Not all Hekatonkheires Mages can even summon 100 arms and only 2 are have been known to summon more than 100. Its a general understanding that the most a mage can summon is 100 arms.