The spies in the Redire Kingdom's ranks weren't from the Tribe of Death. But it seemed like the Redire Kingdom, or at least the people from it, had cooperated with Beltor to betray the human realm in the war with the Tribe of Death.
But the demon that Toz saw in Treblor Forest was neither from the Tribe of Death nor the Tribe of Blood. To Toz, it looked like there was a fourth party. One that Anaster was part of. And that acted like a middleman between the Tribe of Blood and Beltor and the Tribe of Death. But without the Tribe of Death knowing about the Tribe of Blood's intentions.
It was messy, unnecessarily so, in Toz's opinion. But at least it didn't look like Anaster's faction didn't have any intentions of joining the battle. They had done what they were supposed to or whatever. That was why Anaster had retreated through the portal after his fight with Toz. There was nothing left for him to do on the battlefield.