You get after clearing out the leader in The house of bones. Which is quite easy if you learn the game mechanics. You can take out every single enemy on this map one by one without alerting new enemies to the battle.
Simply have all three team members equipped with silent weapons. And let the member with bonus critical while undetected shot first, while the two other shoot also after, this should kill every enemy except the leader in one round. causing the battle to be over even if the enemy dropped is some what close to a oblivious enemy. Enemies do not react to dead bodies outside combat neither do they move around if they aren't part of the combat, so you can just sneak closer to the next enemy and ambush them one by one.
This of course assume that you have returned to the ark at least once to upgrade your silent weapons and add any addons to them or you might fall short on the damage forcing you to deal with enemy rounds and additional enemies and the risk of getting hurt. Magnus will join the party but the game has a limit of 3 party members so you have to pick your team from now on.