Floor Leveling Compound
It depends on why you want it level ie..... to tile or just to make it level.
If it is very uneven it may use a lot of SLC , this stuff is dearer than gold in Ireland !!! If its bad you may be able to either dig it out or screed over it depending on how you are fixed for having a decent thickness of screed , you may need to consider insulation. Do you currently have high heat loss through the floor or dampness or wet coming up? It sounds complex but like most diagnosis knowing more detail would be helpful and would decide the depth of the problem if you pardon the pun.