Hysteria is Valkyr's Rage Mode - we teased and released her as a Beserker frame and we've iterated on Hysteria a couple of times since release. We are returning to this power to reign in its issues but still thematically invoke her ultimate rage.
The relationship between time spent in Hysteria and energy & incoming damage has changed in two ways.
Firstly, the longer you stay in Hysteria, the more Energy it'll cost you and (but the energy drain ramp up is capped).
Secondly, while in Hysteria you are still invulnerable - the current Hysteria indicator on the UI tells you how much incoming damage you've absorbed and mitigated. The longer you stay in Hysteria, the less effective the mitigation of incoming damage is on the 'End Hysteria' phase. Ending your Hysteria session should be done with care - ensuring no enemies are in the range of Valkyrs Hysteria Aura (20 meters max, they have to be able to see you), or you will take the damage. How you choose to dispose of these enemies is up to you - either kill them, or get a safe distance away to deactivate Hysteria.