The True Demon Lord is the one of the ultimate forms of existence. It is one of those Titles that can also be considered a race. This is not to be confused with the Ten Great Demon Lords or the Eight Star Demon Lords, which are organizations.
It is extremely difficult to obtain. Even those who have lived for many eons are not guaranteed such status. Orc Disaster would have become one, but was stopped in the middle of his transformation. Thus, the Demon Lords watch each other, in the hope of preventing others from evolving above them.
To ascend one must first fulfill the requirements. Following the requirements one will enter the Harvest Festival and a personal transformation. Once the transformation is complete a new True Demon Lord is born. True Demon Lords may receive an Ultimate Skill. The Awakening of a new True Demon Lord can be sensed from anywhere in the world, meaning everyone will know when a new True Demon Lord is born. Also, direct subordinates of the newly awakened Demon Lord, meaning those subordinates that have a soul link with it, will receive a gift that makes them stronger and more useful.
To become a True Demon Lord one must first possess a Demon Lord's Seed then collect a vast sum of souls. The amount of souls needed for a normal ascension appears to be 10,000, as Rimuru and an alternate Clayman both prove. The souls do not have to be directly collected by the one partaking in the ascension, but rather as long as the deaths are conducted by their intention the souls will be collected.
Rimuru is able to awaken qualified subordinates through his soul corridor, which was established through the special effects of Gluttonious King Beelzebub's "Food Chain" ability. This method, however requires ten times the number of souls Rimuru himself had to use, namely 100,000. He is also able to awaken other qualified demons by transmitting the energy of 100,000 souls to them, by briefly establishing a soul corridor with them.
It is possible that the increased amount of souls needed for Rimuru's subordinates are due to the fact that they may not possess a Demon King Seed, but merely possess an affinity towards becoming or forming a Demon King Seed.
The Harvest Festival occurs once all the requirements are met and the one to ascend agrees to begin the evolution.
Some will lose consciousness or enter a hibernation state, while others will be able to maintain their composure. Others will struggle to control their new powers.
During the Harvest Festival the one ascending undergoes a transformation in which they can physically transform and gain a tremendous amount of magical power. Additional abilities and Ultimate Skill can be obtained during the transformation. There is also the possibility of a significant change in personality.
Additionally the subordinates of the one who ascends will also receive "gifts" of power or ability.
It has been noted by Rimuru and Guy that this is the final form of monsters under normal circumstances and no matter how many souls they gather there are no ways to evolve further, however, naturally, there are exceptions to this rule.
Rimuru's power increased dramatically after awakening his and Guy's servants as True Demon Lords due to his Food Chain's Soul Corridor. While it's unknown if Guy's subordinates would count, as he'd only need to briefly establish a Soul Corridor to evolve them, Rimuru maintains a permanent Soul Corridor link with his own subordinates, this means that his power increased in direct response to their evolution and put his power level in the same league as those of the True Dragons Veldora and Velgrynd. Furthermore Rimuru was able to evolve into an Ultimate Slime by absorbing Veldora's energy body while leaving his core intact, transforming him into what can in theory be called the 5th True Dragon. Continuing even further, after Wisdom King Raphael evolved into Manas: Ciel, she upgraded everyone's abilities to their utmost peak for the time and Rimuru was finally recognized as a Great Demon Lord by the Voice of the World.