The Rogue Trio part of the Desert Rogue Minigame, is the only mob in the game that you are the only person who can defeat the one you find. Found in the Desert when digging up Purple Graves. After defeating the Rogue that you found the grave will become normal coloured again. The Purple Grave has now moved, when you find the new one you have a chance at finding a Crypt giving loot and thieving XP. You won't always find the Crypt right away, it can take many tries, but the good news is the loot scales with how many tries it takes to find it. There is also a rare chance when digging at the Purple Graves to spawn a Earth Golem (not locked to you).
Weaknesses
Each of the Rogue Trio is weak to a different attack style.
The Rogue Warrior
is weak to 
Fire damage, so you'll want your Staff with some Fire or Lava Orbs and your Magic Armour for them. These are the only of the Rogues that can be safe-spotted using the left walls of the map.
The Rogue Archer
is weak to 
Slash damage, so you'll want either your Sword or Scimitar and your Melee Armour for them.
The Rogue Mage
is weak to 
Archery damage, so you'll want a Bow, some Arrows and your Archery Armour for them.