Ring of Endearment

Ace's mind instantly raced, 'An object and a promise?' 

The very next moment, it finally dawned upon him as he looked at Eva's index finger and blurted, "That storage ring I gifted you, right?" 

Ace instantly remembered the storage ring he first gave her, and it was exactly with that ring Eva tricked him by performing the devil race's ritual and bonding her life with his. 

Although he didn't suffer or lose anything. Eva's life got bonded with him, and if he dies, she will also follow suit while he would suffer no such causality if something happened to her. 

Furthermore, it was also the symbol of marriage in Devil Race. But he never told Eva that he already knew about this ritual and marriage.

However, the ring had vanished after she woke up, so he didn't have much about it. Moreover, he didn't have time to think because he was too busy focusing on other 'matters.'