As you've studied magic, your studies have brought you to learn about Fiends, Specifically Yugoloths. You through research have found a way to call apon a Yagnoloth, and through him you can make contracts to summon minions under his control to fight for or otherwise use their skills for diplomacy safety or protection. You are developing a relationship with Your Yagnoloth and it might not trust you enough to make bigger contracts only allowing you the smaller ones until you develop a better relationship with it.The yagnoloth is a fully sentient creature and does remember your encounters if you trick it or give him a bad deal, you may summon Different yagnoloth but it might have a personality more hostile or may be more greedy (To the dms discretion), The deals the Yagnoloth makes are completely up to the demon but are based on your relationship with the fiend and the base cost of the Deal.

Summon Yangoloth

Casting a special planar ally you Summon a yangoloth that is a mercenary lord willing to trade some of his army out for Gold or souls, the selection of yugoloths is very limited at first but grows as you progress. The deal has to be agreeable to the yangoloth as well, Offering a bad deal might anger the yangoloth enough for him to leave.

Contract: Mezzoloth and Merrenoloth

You may discuss deals involving Merrenoloth and Mezzoloth regarding hiring them, the Yagnoloth may put caveats on the contract of which you must comply with to summon them (dms Discretion)

Merrenoloth:50gp per day 1gp per mile traveled 50gp extra if killed

Mezzoloth:50gp per battle 25gp extra if killed (is summoned for battle doesn't travel with the pc, may be summoned as a bonus action if deal is made)

Contract: Dhergoloth and Hydroloth

You may discuss deals involving Dhergoloth and Hydroloth regarding hiring them, the Yagnoloth may put caveats on the contract of which you must comply with to summon them (dms Discretion)

Dhergoloth: 75gp per battle 50gp extra if killed

Hydroloth: 100gp per battle 25gp per secret stolen from target 50gp extra if killed 

Contract: Canoloth and Oinoloth

You may discuss deals involving Canoloth and Oinoloth regarding hiring them, the Yagnoloth may put caveats on the contract of which you must comply with to summon them (dms Discretion)

Canoloth: 100gp To summon 10gp Every day 100gp extra if killed (will not travel, must guard specified location)

Oinoloth: 250gp Every day 50gp every bringer of plagues used 200gp extra if killed (must stay in area of 5 miles from where it was summoned)

Yagnoloth Training

Yangoloth leadership and cunning has rubbed off on you, as well as your reputation in Gehenna, allowing you to recruit soldiers more cheaply and lead them into battle much more effectively.