Viktor has left that mystery for others to puzzle out. From what he knows, they had knowledge to get around the wards, but not of the summoning ritual itself. Istredd is right that they will not be able to depend on their further intervention. Anything the Summoned want, at this point, they'll need to take themselves.

And he's right, too, that those in Thorne need to navigate a new king and a new high mage. All the more reason why Viktor is counting on discovering his own version of the ritual, hopefully one with fewer environmental impacts.

It is difficult to talk about what drives him without revealing the selfish nature of his research. He maintains, however, that untangling the ritual would be beneficial both for their number and for this world, for all of the reasons he's already spoken on.

There is more, though.

"I have obligations to my home. To remain here, and leave my work unfinished...it would be unconscionable."