In MH mode, when you pickup a weapon it randomizes a "relic" to be used with that weapon.
Most of these relics can be changed and picked up around in maps.
However, there's 1 special kind of relic that you can never pickup, you can only obtain it in that random relic selection on weapon pickup, but there's a low probability of getting it in the picked up weapon.
This relic is called "The One", and it makes your weapon able to hold 3 relics at the same time (although you have to pick those 3 ones by yourself around the map).
There's another limitation: you can never change the picked up relics, so once you fill it up, it will stay with the same 3 relics no matter what you do (until you either drop the weapon or get killed).
All the weapons (except super weapons and nukes) can have it, but the probability of actually getting one is low, therefore you get one with a bit of luck.
Personally, once I got that relic, damage+fast+infinity was my favorite combo
