Rex used the chance to use his sovereign necromancy to summon all of his minions, and he was lucky that no one was looking because they were fighting and taking cover.
It was also the perfect timing to ambush them all at once.
The ambush didn't give any warning, and that was the point.
The group first knew something was wrong when Aisella said, "Something's wrong," two seconds before the constructs came through the passage openings. By then, the positioning was already set.
Twelve goblins came through the south passage, six orc constructs came through the east passage, and the wolves were spread out around the edges of the chamber in a way that put pressure on the back and sides of each formation at the same time.
The effect on a group that was already fully committed to fighting a huge apex creature was exactly what Rex had planned. Three different things that needed to be done right away, and none of them had been there a second ago.
"MORE MONSTERS INCOMINGGGGG!!!"
