Even without the proposal of the Grand Inquisitor Valon, he would have taken action, and he expected to acquire a new Shadow Domain there.
When the Oracle of Deep Forest Valley, Laurantice, waved his magic wand, a portal materialized before everyone. He said to the Lord of Deep Forest Valley, "This is the portal to Lake Seber, connecting to the Sea Elf Village. The task of blocking the retreat of the Koutao Fishmen falls to you, Lord. I shall accompany Duke Gaven and Grand Inquisitor Valon to Siren Valley to see the situation there firsthand."
As previously mentioned, the closest to the vanished Siren Valley was not Bow Valley, but their own Deep Forest Valley. During the war between Bow Valley and Siren Valley, although Deep Forest Valley was nominally neutral, in reality, they were implicitly condoning Bow Valley's actions.