I used to have eczema all over my butt cheek and now it turned into a dark tone scar, will it ever fade or it's going to be permanent?

Likely fade. Most often, after eczema or inflammation in the skin there is some change in the pigmentation. For some, it is lighter (hypopigmentation); for others darker (hyperpigmentation). This almost always fades in time, though it can last a for several months in some. Generally, good moisturizing and strict sun avoidance are helpful. In some cases, prescription bleaching creams can speed this up.