Becoming of Rank
To become an Infernas was a great honour and privilege, with a great level of difficulty to become, as the method used wasn't always uniform.
The first infernas were given the position due to their close relations and long history with The Vulkan. Afterwards, however, it wasn't very set in stone.
Some later infernas earned the position due to old age, as ancient Sol'Mundian culture always favoured the eldest of every family as leaders. As such, the elderly had better chances of achieving the position, typically those with the mental maturity of a 70 year old human.
Another method used by some was military prowess and merit, as the order was a military order as well. War had always been a harsh reality in the galaxy, and sol'mund was no exception. War heroes were always told of in sol'mundian tradition.
Another way to gain the position was through artisanship. The vulkans were valued as artisans as much as they were as warriors, and this also worked into gaining the position as an infernas.
Then there were those who gained the position through their scholastic education in sol'mundian lore and the ways of the ancient Temvars. These ones in specific were priests to be revered. They usually attended universities and colleges.
And last was through a display of their mastery over The Force, and impressive mastery would often result in them being ascended into the rank of Infernas.