A Demi God「半神半人, HanShin HanJin, lit. "Half God Half Person"」is a title granted to someone who has surpassed a True Hero.
Unlike Heroes, who are flesh and blood humans of high ranks and almost infinite lifespan, Demi Gods are spiritual existences who do not face physical damage and can quickly regenerate as long as their cores remain intact.
As spiritual beings, they are also capable of moving within "Suspended World" which is a skill that stops time for all matter and anything related to physicality, even light.
Thus, "Demi God" the highest form of evolution for a human known yet and the form is so powerful that it rivals even the likes of True Dragons. Thus, this evolution puts them in the highest echelons of the World.
There is no known general method for a Hero to evolve into a Demi God since the only living examples had special circumstances (Chloe having an unknown number of time leaps, and Leon being reconstructed by Ciel) that caused their evolution (unless the requirement for evolution is a special circumstance itself).
Footman immediately dealt a fatal thrust into Leon's chest after acquiring Vega's seed (and Evil Dragon Lord Azi Dahaka) and the dead Kazaream's energy. Since Leon was still just a True Hero, a mortal, he could not recover from that on his own.
However, Leon's core was apparently damaged, and according to Ciel, he would have to be reconstructed as a being similar to Chloe. Rimuru was already having trouble processing the explanation Ciel had given on how Diablo could further utilize his Temptation King Azazel, and so didn't care about any details on how to fix Leon.
Since it was Ciel, Rimuru had full faith anyway. And he already speculated an increase in stats based on how Ciel has been doing things till date, so he was fine with it since Leon wasn't an enemy.
But to his surprise, Leon's upgrade was far greater than he expected, as he was turned into a completely different being, a Demi God.
It is unknown when "exactly" she turned into a Demi God, but it was of course during one of her time leaps. It was hinted by Ciel in the previous chapter that Chloe was a Demi God, but was confirmed in Chapter 215 when Guy noticed Chloe could still move even after Velzado used "Suspended World" with her Patience King Gabriel.