Evolution in Dark Summoner is a straightforward process, performed by sacrificing a Rank A or Rank AA monster to a second Rank A or AA monster with the exact same name (e.g. a Rank A Reaper Kilviny to another Rank A Reaper Kilviny).

Description Evolution turns the powered-up Rank A/AA monster into a Rank A+/AA+ monster, with much higher starting stats and higher stat gain per level.

Of major importance is that the evolved monster's Level will be reset to 1 after evolution, so it may be temporarily less powerful than your original monster.