You have not mentioned if the ‘unevenness’ is very deep . If it isn’t I would score it using and angle grinder to give it some purchase and clean the area thoroughly with an acid if necessary then use self-levelling cement – after pouring the cement using a level screed board to remove any cement – Hopefully, you intend covering the area with carpet or floor tiles
Follow the directions on the bag of SLC – if the instructions tell you to lay a minimum depth that you should lay and you don’t do that you may have flaking and crumbling where it fades to nothing.
If you can afford it and you have the depth you should apply it to the whole area and make it around an inch deep (or whatever they recommend) – put edging where you do not want it to spread and remember it truly is self-levelling so if your existing floor leans to one side then the cement will end up being deeper on that side.
I have laid about 20 square metres and it is a remarkable product – you can also see YouTube video of people laying it.