This achievement is gained after completing all the quests in the Underfoot.

Complete the achievement "Mercenary of Brell's Rest"
Complete the achievement "Partisan of Brell's Rest"
Complete the achievement "Mercenary of Fungal Forest"
Complete the achievement "Partisan of Fungal Forest"
Complete the achievement "Mercenary of The Underquarry"
Complete the achievement "Partisan of The Underquarry"
Complete the achievement "Mercenary of The Cooling Chamber"
Complete the achievement "Partisan of The Cooling Chamber"
Complete the achievement "Mercenary of Kernagir"
Complete the achievement "Partisan of Kernagir"
Complete the achievement "Mercenary of Arthicrex"
Complete the achievement "Partisan of Arthicrex"
Complete the achievement "Mercenary of The Foundation"
Complete the achievement "Partisan of The Foundation"
Complete the achievement "Mercenary of Lichen Creep"
Complete the achievement "Partisan of Lichen Creep"
Complete the achievement "Mercenary of Pellucid Grotto"
Complete the achievement "Partisan of Pellucid Grotto"
Complete the achievement "Mercenary of Volska's Husk"
Complete the achievement "Partisan of Volska's Husk"
Complete the achievement "Mercenary of Brell's Temple"
Complete the achievement "Partisan of Brell's Temple"
Complete the achievement "Mercenary of Convorteum"
Complete the achievement "Partisan of Convorteum"