Her name is Feniss and she's here to tell him that Leareth says he had no involvement in either recent incident and, uh, to her knowledge cannot possibly have destroyed k'Treva, and it's in theory possible she wouldn't know if he had planned the Changecreature attack but it would make no sense with what she knows of his other planning. She wasn't chosen for being ignorant of a lot of Leareth's operations and thus unable to give things away; she does some administrative work related to the god-research side of his organization, mainly, and she's not closely involved in his military planning, but is pretty sure that she would know if he was staging large-scale troop movements and he isn't. Until yesterday everything was routine and happening at the pace you would expect if he planned to move in a year or two - though she didn't know the exact planned date - and even in an organization with a lot of compartmentalization and secrecy compulsions in routine use, she would have noticed if some of her friends were suddenly inexplicably busy even if she wasn't cleared to know what they were working on.
(Reading between the lines, Vanyel could learn quite a lot about how Leareth's organization operates - and its weaknesses - by asking her the right questions. That's not the main message she's here to convey but it's deliberate, that he wouldn't have chosen to send her unless he was willing to take that risk and wanted Vanyel to know that.)
Leareth instructed her to convey that another world existing changes everything and he intends to take a step back and re-evaluate, and also that's...self-evident to her...from everything. Leareth took a thousand years to move on this plan at all, even though it was one of the first he thought of. She thinks he probably took everything into account correctly on how long to spend looking for alternatives, but some of the people she works with think he should have moved sooner. But obviously waiting is going to turn out to have been the right call if there's another world and there was no way to know that until now! It seems pretty likely the whole invasion plan will be called off and they're going to pivot the organization toward work related to the other world, though of course it's premature to get hopeful about that when they don't actually know for sure if two-way travel is possible.
She thinks it's not really that surprising that the Star-Eyed Goddess (or other gods) would be willing to kill several hundred people to push the war to happen now, especially if They thought Vanyel might be on a track where he would otherwise end up allying with Leareth. The gods kill thousands of people all the time in the downstream effects of Their actions, just in less obvious ways. It's mostly surprising and confusing because it's blatant, and you would expect it to be costly to Her and possibly to cause the kind of chaotic ripples in Foresight that would make it harder for the gods to precisely nudge events happening now, and also it seems like...overkill? And like a plan more obviously-to-Vanyel within Leareth's capabilities would have been more likely to convince Vanyel it was his doing. And in general she thinks they're probably missing part of the picture.
She wants to convey that Leareth was aware the kidnapping was incredibly escalatory and would be hard to step back from, only made the decision at all because he was worried there might be more destructive interventions if the god-agent was left in Haven, is not sure it was the right call even in expectation, and is probably willing to make even very costly concessions now if it will convince Vanyel he wants to avoid a war. She personally isn't authorized to promise anything on his behalf, but she can answer any questions he has and can in fact talk about a lot of topics she normally couldn't openly, she works at a high enough level to have agreed to standard secrecy compulsions but Leareth took them off for this.
She does have a way to get messages back to Leareth, though it's a one-time-use artifact so ideally it would be one message once Vanyel knows what he wants to say.