These cards are either great at dishing out the -1/-1 counters (in the case of the Mentor) or interacting with creatures that already have them
Sorin, Imperious Bloodlord activates its -3 ability to play your Bloodthirsty Conqueror from your hand on turn 3 after you dropped Starscape Cleric on turn 2
It was one of the first commander decks I built and I havent touched it in a long time
Chaos players will certainly love what this card does
Mirri also helps cover you in the aftermath of an attack, allowing opponents to only attack with a single creature as long as Mirri is tapped, mitigating the damage the Mirri player would likely sustain in a potential counterattack
When you deal a point of damage to an opponent by removing a counter, you also gain a life yourself, causing you to put another counter and go infinite, thereby wiping out all of your opponents