The room Lyda and Pressia are in is a planetarium; they're in there because they need to talk to Arvin Weed.
When Weed walks in, Pressia asks him what side he's on. He tells her he's on his own side.
Weed's a lone warrior.
He also tells Lyda that Partridge had been writing her letters the whole time. So at least she knows he still loves her.
Weed pleads Pressia to give him whatever she has—which is the formula—so he can reverse the Rapid Cell Degeneration in himself, and the fusings in the wretches.
Lyda also admits that she sent the message out to Bradwell and El Capitan without telling Pressia. Whoops.
Pressia hesitates to give the vial and formula to Weed, but she decides to take a leap of faith.
And it seems like a good one too. Weed tells them that Willux killed his parents, and his little sister. It looks like he's out for revenge too.
He tells them he'll send word out to Partridge, and that Pressia should probably take one of Lyda's spears because she might need it.