These contracts can only be completed by Snikch, any Master Assassin, Assassin or Eshin Sorcerer. Occasionally, the four great clans, Clan Mors, Clan Pestilens, Clan Skryre, and Clan Moulder will give out contracts to Clan Eshin and when completed, they provide great buffs and give food and money. A performed action will have a turn cooldown before it can be performed again. Each action costs a varying amount of Food or Schemes to perform, and the agent must meet the level requirement and will need to rest for a number of turns afterward.
You can see it below (click for a much larger version) it includes the starting locations for all Legendary Lords.
No more need to squint at Total War: Warhammer 2 mini-maps in Steam screenshots, the full Mortal Empires campaign map is now available.