It is impossible to synthesize a behemoth slime: they must be scouted on an uncharted island that is inhabited only by Slimes and She-Slimes. The uncharted island is between either Palaish or Xeroph and Domus Isle. Behemoth Slimes have the same ability as normal slimes, but synth two together and voila! You get a King Slime. Also if you synth a sheslime with it you get a, you guessed it, a shebeomoth slime.