First get rid of the death aspect altogether. It alienates people too much.
You can have a separate death tournament or something. The best of the best style.
But normal death causes too much trouble.
Now to the structure of the organization.
The biggest issue is funds. Are you a mad billionaire who is funding this?
Or do you have to actually find a method of getting funds?
Anyway I'll talk about general principles and talk about money later.
City cell is the answer. Each city works separately from the other with a city leader handling the affairs of the city. Cell leaders don't know each other at all.
No inter-city competitions or moving. Just separate city entities.
Second the City leaders at first would report to a single person.
Something like a state leader.
That state leader never meets them in the same place twice, never discusses other members of the cells with other members.
Better yet if you grow big enough split the city 2 and have two different leaders state leaders. Same rules though. State leaders only meet alone with the person above them.
Orders are only given verbally and without anyone but the person giving the order and the person handling the situation.
This is something like what the Godfather does.
Just think that at anytime a cell can be caught so what can you do to make sure that nothing more than that cell is comprised?
Because even if a cell leader rats they only know one name and that is the person above them. But even then they have no means of implicating them in anything.
And much like the proof of loyalty of the Mafia is killing someone.
You will use the same principle.
If someone wants to become a part of the organization then they must murder someone in cold blood, I think if a match is better as it shows strength, with their respective leader as the only witness. That remove the fear of anyone else ratting on the two others.
Anyway as you go up the chain of command the same logic should be applied.
Cold blooded murder. And, if possible, security checks.
Supplying the unfortunate souls should not be a problem. Just random violent criminals should keep you swimming in people to kill.
The important part about recruitment is the screening process.
You won't just let people off the street into your super secretive hideout.
Generally you spread the word around anywhere. It won't matter.
Then you figure out a way of insuring the loyalty and the sincerity of the people trying to join. Here, again, a small criminal test is the answer.
You only let people in after said person commits a certain crime, murder is too much so maybe something less sever but still totally illegal and can get them to jail, in the presence of the recruiter so again none of them can testify or cause trouble to the other.
As you grow and continually test the loyalty of the member then you can start accepting people only based on recommendations.
That is to be even accepted to be tested.
You can still leave the door open to men with outside individuals with extraordinary abilities to join.
And word of mouth is actually very good for this sort of thing.
Nothing concrete and for people serious enough to join it's enough.
Gossip is a potent form of advertising.
You should also have in place a robust system once people are inside. Much like the Godfather. There you can be a cop then turn criminal and rise to become the Don's bodyguard. But there is a testing process involved.
Lastly. Much like fight club. You will start having a cult and people getting involved more and more. You can use the hardcore members for violence and external fighting, the state or local gangs or such, while still keeping a large pool of casual members.
Developing a cult is a matter of money, charisma, and organization skills.
Simple as that.
Now for extra fun. Bribery. Oh boy. The government, every single one, is susceptible to bribery and corruption.
Having a cover of judges, officials, police captains, journalists and other media types...etc can be the difference between a terrorist group and just a harmless gentleman's club.
With enough money and dirt work all those type can be in your pocket.
Then Khorne would be pleased.