The reason is quite simple.
Angel saw, in the vision of the Doomsday Eye, a red ribbon swirling above the blood-stained Sky Dome, a ribbon he had seen before on Frost's companion.
Frost didn't go to the Reincarnation Casket alone; with him went a mysterious hunter named Zorn.
And Zorn carries a mysterious object which is a red scarf, identical to the ribbon in the Doomsday Eye vision.
With this, Angel deduced that the location of the Doomsday Eye is indeed the small space-time of the Reincarnation Casket.
Although Angel determined that the Doomsday Eye occurred within the Reincarnation Casket, he couldn't comprehend why the apprentices of the Nebula Tournament would be inside the Reincarnation Casket?
But now, he seems to understand.
It's not that the apprentices of the Nebula Tournament fell into the Reincarnation Casket, but rather the expedition task released by the Sky Tower has summoned them into the Reincarnation Casket.
Upon thinking of this, Angel's mood grew complex.
