How does unholy might mastery and behead support for spiders works?

On my pob it shows its a huge difference in dmg if my spiders have unholy might but does each spider only get unholy might each time it gets a kill or does it apply to all of them when any of them gets a kill? Also is behead support the same or does it apply the buff to all the spiders at once?

Each spider is its own thing and needs to make its own kills for those effects.

Behead is extremely inefficient to use with them. It will only work if a spider makes a killing hit with its Viper Strike ability. It won't work if it kills by poison damage or some other means. One spider at a time only occasionally getting a random buff, even if it's one of the AoE/allies buffs, from a rare mob kill isn't worth the gem slot.