For my Power System, Essence, the most complicated part of it is definitely gonna be Ultimates (inspired from Reality Marbles from the Fate Series, and also Domain Expansions from jjk but they both have similar premises)
Ultimates are the pinnacle of power in the world of Essence, a rare occurrence that only the strongest can attain. It is a projection of the user's Inner World, a manifestation of the root of their soul, or their true self.
There are two types of Ultimates: Boundary (Bound or Boundless), and Open-Field. Boundary encases the Ultimate in a boundary outside of the real world, not letting anyone inside the boundary escape, in a Sphere of Influence, where it acts as a limiter to not let the Inner World spread out to the real world, while Open-Fields reject the idea of a boundary or Sphere of Influence, and spreads the Inner World throughout reality, expanding at a constant rate, but letting opponents escape its range.
When Ultimates are casted, they start with an Opening Ceremony, where the Inner World and Sphere of Influence (Only for Boundary Types) is created. However, once they unleash a hand sign and a chant, they unleash the Universal Effect unto everyone. The Universal Effect is the effect wherein the Inner World is imbued with Essence, and then imbues it into every person inside the Boundary, therefore making them affected by the Ultimate's ability.
The only way to stop an Ultimate, is to interrupt the Opening Ceremony before the user unleashes the Universal Effect, casting your own Ultimate which results in a clash between two Ultimates, and an inherited technique that is a well-kept secret passed by the last members of an extinct clan to their peers and allied clans, but I'm not gonna reveal that for now.
I'm definitely not gonna elaborate on the Types of Ultimate clashes, because it would make it too complicated to even fit here. But there are three types: Boundary vs Boundary, Open-Field vs Open-Field, and Boundary vs Open-Field. Just think of a Boundary-Type as a rock and Open-Field as a crumpled piece of paper that constantly uncrumples itself and expands.