The title of a Mortal God is assigned by the Congregation of Approval, one of the two departments of the Electorial Congregation.
This special title is only given out to the best of the best. They have to overcome some very hard and mostly deadly tests, designed by the Congregation of Approval. It is a mixture of chemistry, physics, a little bit of alchemy, magic, combat and decision-making.
After beating the test - and still alive to recieve it - the applicant will be assigned the title of "Mortal God" and given the questionable privilege to enter the Spiral.
The Congregation of Approval tests everyone, even Trolls can apply.
After achieving their title the Mortal Gods is their first task: visit the Spiral. See what you are dealing with and then develop plans and ideas. That can include smith special weapons, lead a charge into the Crimson Chamber or develop alchemistic substances which help to sustain the defensive capabilities of the Black Fortress. You as a Mortal God are responsible for your actions and you have to deal with your tasks. If you can't fight, then you can think and chop chop, think fast!
A Mortal God is not only a fighter. He can be a paladin, an archer, a wizard, a Vael or even a scholar. Sometimes Mortal Gods join forces with each other so they have better chances of survival or fulfilling their goal. A paladin blocking enemies while they got obliterated by the wizard behind the paladin? Sounds like a good trade. And if one member gets hurt, maybe the alchemy knowing Mortal God has a potion with them.
The direct benefits from the Electorial Congregation are simple: resources. Be it coins in every currency the Golden Hoard Capital and Investment Management has to offer; manpower or wood, stone, iron, food or knowledge.
The second direct benefit is the crest. With it a Mortal God gets free food, drinks and a room in every tavern to a degree. They can order horses for their purpose, they can command armies and they even can give orders to mayors or landlords. Barons, kings, queens and everyone in this state of royality is an exception.
And the last benefit has two sides: the first is a really good monthly income from the funds of the Electorial Congregation and, if you survive your twenty years of employment, you can get a house in a location of your choice (except palaces and the Emerald Planes) or the equivalent in a currency of your own choice. The Electorial Congregation even contracts an architect and the builders for the house.