Ace summoned Cyrus because he had a feeling that Cyrus might not trigger any heavenly tribulation. Nonetheless, he was ready to send him away if he sensed something because he couldn't afford to draw any attention to himself.
However, if nothing happened, then this means he would have a powerful trump card that wasn't restricted like him, and he wouldn't have to be so passive.
"Oh… where are we, big bro?" Cyrus asked curiously and with a hint of excitement as he observed the stone chamber. He'd been in the thief's space for quite some time now, so getting out made the little guy quite excited, especially when he thought Ace was taking him on food-hunting.
Ace didn't reply and warily felt his surroundings. When nothing out of the ordinary happened, he sighed in relief, and his eyes shimmered with elation. This meant he didn't have to worry about Cyrus triggering any supernatural phenomena.
