If its a 2D RPG then you are surprisingly more flexible than its 3D counterpart. If you are working with just simple sprites then you are only limited by your desire and skill to animate it.
There won't be any right answer here as this is more a graphical design choice than a technical question or limitation - due to that I would recommend making this a wiki entry for "ideas for morphing" or similar.
My idea would be to play on your games title and theme: Necro-Monster.
Seeing as "necro" signifies the dead and corpses you could have the character sink into the floor or crouch into a black, death like mass (giving you a set starting point for all charater transformations to keep things simple). After a short pause they "uncrouch" into the monster shape. The monster form would then shake off the black-death mass material or it would simply fade or slide off depending on your desired graphical complexity.