Sky Sanders is married, and lives in a relatively unremarkable apartment. A nice apartment, but one of thousands. There's no real need to infiltrate, just knock on the door.
"So you want to find out about Valterre so you don't get instakilled when you fight Spar."
"That's pretty much all of it."
"I'm going to make this quite clear. You're going to die if you do that. She will kill you, protected or not, either at her hand or the hands of her team. You have to know that, right?"
"I've been made aware. I'm going to fight them anyway. It's too important."
"Alright. Now, how technical do you want it to be...."
In summary..
Valterre has a guaranteed instakill effect. This can be blocked with the Death Ward spell, or an accessory that gives the same effect. You probably want the accessory, as Death Ward is removed when someone is knocked out. KO + revive = vulnerable once more.
Valterre also introduces no less than a four-times elemental weakness at the time of striking, to a random major element. One of the six main elements. Four times. If you are triple-weak to an element, and hit by the element, you are killed, regardless of hit points. Double-weak is double damage. Single weak is 50% more. Triple-weak is Death. With a four times elemental weakness, you must be ++ or better in that resistance to survive.
This effect is repeated twice more, hitting the person with three different elements at once. If you aren't double-resistant to all the six primary elements, it will find the one that kills you very quickly.
..
..If you are not killed, it simply hits you with relatively extreme damage. It doesn't ignore armour, despite looking like an energy blade, but it still does a lot of base damage. Enough that even if the elemental instakill doesn't happen, you're likely to get hit with the strongest blows you've ever faced. Still, it should be survivable.. for you, assuming you aren't at a buff disadvantage. Trence should have a chance. Willis will probably want some bonus HP. Alice is about as tough as Trence, with the barding making up for no dragonscales.
mmm. And magic resistance should help, given that even if you double resist it, magic damage is going to be doubled, making points on armour count twice as much. And the big weakness remains.. the sword can only do this once per turn.