Here's a thought.
Eb summoned them on accident, simply by drawing their attention by inadvertently messing with the localized flow of time.
The universe yawed slightly in his direction as he
did, a subtle bending of light, a minor wobble of
gravity, a shudder in the very ground as Ebenezar
drank power from the earth itself.
Bending light?
Universe yawing toward him?
That's the kind of behavior one sees around a black hole.
And as we all know, dense gravity can affect the flow of time. Dense gravity that's strong enough to bend light would be way more than enough to cause gravitational time dilation.
So maybe it was less that someone sent them, and more that they were already in reality, and Eb accidentally did enough, between his power and his fear, to draw their attention.
We're so busy trying to fit the attack into the plot, when it might later be revealed that it had nothing to do with the bad guys, and everything to do with Eb losing control of his power, as others pointed out.