Might as well explain. 

The swords work by gathering different energies and then pilebunking them into one form. Now, the problem here is that the source and purposes for the energies are different and thus have a little of what they used to be engraved in them - not enough to reproduce the full effects of the parent spell, but enough to remember where it came from and develop an immunity to via inherent recognition of a spell.

In short words, for everyone that ever released a spell or ki blast in this fight, the swords have become immune to [i]each and every one of your spells[/i]. How? The moment you use a spell on it, it breaks itself apart until it has a signature that can match the spell, fuses with it from there, then overloads it from the inside until it's completely broken down - this process takes less than a second. Even [i]Event Negation[/i] requires some form of recognition from the spell caster, otherwise it wouldn't even be useable, so naturally it falls under the domain of 'immune against'.

The only way for the swords anti-whatever system to fail is if it encounters something that has no connection whatsover to what it absorbed, but even then it still has a sh!t-ton of power behind it. 

Yeah, sounds a little OP, but you gave Meruin more than half a hundred posts to prepare for it, what did you expect? :D

Anyway, it does have a drawback: they can't move farther than 7 meters away from Meruin, and as powerful as they are, there are only 7 of them.

EDIT: TL;DR - They are antibacteria for everyone that contributed to its making.