Prisoner + Primordials inflict silver debuffs and disables passives AND grants your team gold buffs like Ascension and Crit Strike..... KP removes 2 gold buffs from enemies. Basically it's a set up to remove boost and inflict things like Perfect Corruption or Exhaust at the same time as disabling passives on enemies while at the same time boosting your team.
It works really well, for first strike. But if it doesn't get first strike, it alone can't save a team.
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Doesn't HAVE to be a 6 set, but since nothing is going to add a buff, why not a 6 set? It would specifically prevent things like energy or vampire, or sustaining, or fortune talismans on the character as well.