As you can tell from the comments below, people are saying that the ending is bad. And I personally dont think it was that bad but I think it was okay.
!!!SPOILER BELOW!!! (Very detailed?)
.
.
.
.
.
.
.
.
In the end, Sung Jin-Woo was up against a monarch/sovereign that was the Dragon Emperor. When he defeated the Dragon Emperor, Jin-Woo wished to reset time and start the battle all over again so he can save the ones that died during the war. He made that wish to these Angel-like beings(forgot what they are called) and that wish was to use the Chalice of Rebirth(the time resetter). Though, that wish had to be done with a cost. Thus, the cost was that everyone will forget him fighting those creatures, the gates, power, and etc. And honestly, it was sort of sad. However, there are side stories. The side stories are much more happier and focuses on Jin-Woo's school life and his future life in which he is married to the other S-rank hero(Cha Hae-In).
P.S - They had a son(son's school life is included in the side stories as well, but only for 3 chapters) Forgot his name.
I too was confused when this was said in the novel, but what I meant is that Jin-Woo will not go back to an E-rank. Only his appearance will change, some of his shadows will disappear(like iron and someone else) but his powers will remain the same when time turns back. The only thing that was changed was that there are no more dungeons or powers existing(except for Jin-Woo's powers). It turned back to how life is in real life. Also the fact that all the humans act as if they never went through the hardships of those dungeons.
New mask? Doesnt matter, let's appreciate the artist's work ╮( ̄▽ ̄)╭