Get ready for a whole novel
Like mentioned in her main reference, Stevi is technically a sentient curse but with god-like strength and thus referred to as a god/goddess or a devil depending on who you ask.
Stevi is the “child” of the god of death, Ammit. Gods have unusual ways of reproduction and it’s a bit more of creating. Ammit created a curse essentially and sent it to earth via meteorite sometime around 2300 BCE. In this weak state, Stevi could only possess inanimate objects through harsh contact or creatures of the female gender. She could not control creatures very well in the beginning and her presence would make the creatures bloodthirsty psychotic killers. Especially among humans since that is what she wanted to get was a human body. Her victims would end up dying from someone defending themselves. Stevi would then possess the murder weapon if there was one and the clothing of the woman she was inhabiting if the weapon was not a tool. Because of this she ended up trapped in a brick and left in a pyramid for thousands of years.
Even in the weak state, she was able to very slowly accumulate power. 19 years before obtaining a human form, she was able to emit radio waves (specifically very low frequency waves or VLF waves) mixed with spiritual waves, which demons and soul eaters would be able to sense, hoping to attract anything. Her emissions caught S.H.I.E.L.D.’s attention, and a group of agents went in to investigate. As fate would have it, a female agent happened to make contact with the limestone brick that Stevi was trapped in. Stevi was able to possess the agent who then also went berserk and attacked a fellow agent. The other agent knocked her out and from then on Stevi was unable to take any control over her new host.
A year or so later, Stevi’s host ended up pregnant and Stevi decided to be the baby since it was a female. So the baby did not have its own human soul and instead Stevi was its soul. But through birth, Stevi lost all of her memories and as far as she knew, she was born a human baby.
There was a reason for Stevi being placed into the world instead of thriving with her real mother. In order to ascend to godhood, Stevi had to prove herself in a test. The whole finding a host until being able to form her own body/form was part of it. The rest is to destroy a rogue neighboring universe that threatened to eliminate the universe that she is in. If she was able to complete that, then she officially ascends to godhood and her current universe becomes her domain including any dimensions within that universe. The power needed to even fight another universe is something that must be found.
Stevi obtains power through eating evil hearts (along with souls) of other beings/creatures. The darker the heart, the more power gained. Hearts of very strong beings also can provide a lot of power. An ability that Stevi had even in human form was the ability to attract beings who she could gain enough power from to be able to complete her test. But this also attracts another type of being: a worthy candidate of becoming her “herder”.
A herder is a servant of certain types of gods. They gain power close to their master’s, which does allow the herder to backstab their master and rule over the god if they choose to and are able to catch the god off guard. The herder’s main job is to help the god fulfill their job. In this case, Stevi’s herder would need to help provide the hearts of evil beings so that Stevi could eat them since that is her job. Stevi’s godly ability is to convert matter into energy in order to help prevent a heat death of the multiverse, which she does by eating hearts. The herder’s job is to also take care of the god when needed.
Because of the potential of the herder backstabbing her, Stevi has another ability that she had even in human form. It is incredibly situational, however. Stevi’s has shadows and lights that dance around in her eyes. It can only be seen if the being looking at her eyes know they exist. If a being looks for a few seconds too long, their willpower will be completely wiped out and it allows technically anything to control the victim. But this only works if the being knows about the lights, otherwise nothing happens.
Now that Stevi has obtained godhood, she has nigh omnipotence within her domain, meaning she has many powers but she is not all powerful. For example, she cannot turn energy into matter, nor move herself forward or backward in time. She is also limited due to her creativity and keeping Theo not angry with her.