A freebie contract followed by a ritual would be how I'd run it. Concerning contracts, if the price is right they will kill literally anyone that isn't a legitimate monarch, but as they are a rather professional organization they wouldn't be doing normal, run of the mill hits. I'd say make a mini-adventure the PC and set him to the task of assassinating a minor noble or prominent merchant, the kind with lots of low level guards.
As far as an initiation thing goes, look at what they did in the (good) Assassin's creed games: You would lose your ring finger to accommodate the blade, jump off a huge cliff, and recite a code of honor to your new brethren. In 2 this was scaled down to marking the ring finger with a brand, jumping off a building (since cliffs were scarce in Italy), and making the ritual more festive. Then they took all this and threw it out the window and made money grab garbage. Rant aside, just mimic this in the flavor of the mantis' style; I'm not an expert on them but you have them receive some sort of hidden mark so there is no way to mistake a member, prove their bravery in the face of almost certain death, and finish with a recitation - I'm fairly sure Crimson Throne had something for that in one of their splat books or even in the 2nd/3rd module.