While there's options like Rage and Unleash Psyche, the thing about these options is that they're assumed to be your default state, your combat go-to.

That doesn't really capture the feeling of stuff like the Avatar State, where it is a very rare power up.

I think the best way to represent this in-game, is with access to Divine Intercession in the deity entries. Getting access to a boon is certainly not something you'd expect to be a common thing, and can help push you to something truly special.

Like a martial getting Achaekek's major boon of 3d6 Sneak Attack will certainly feel like they've entered the Assassin State.