Castlevania II: Simon's Quest

On paper it is what Legacy of Kain: Soul Reaver is — dark, oppressive atmosphere, horror setting and single-player focus — though the resemblance is mostly at that level. Both also lean on exploration and discovery. The distance is in visuals and tone — its own ground is 2D presentation.
It predates Legacy of Kain: Soul Reaver by 12 years, with what that implies technically.





