What about a demigod who has two forms and to sets of skills. In his First form he is just a big pile of bones and in his secound he is a giant skelton.

In his first form as more and more bones get added to the pile as he kills people he grows in sizes till eventally he gets to the size of the rook.

His skills in the first form should be

slot 1: mimic demi god = he Changes into a bone form of a demi god of equal size for a short amout of time when this skill is maxed he can stay in the form of another demigod for as long as he wonts and wonts he has got a upgrade that will allow him to sample the creatures form for future use on the right and left were the general comands are that becomes the slots that hold his shapes he can change into if he gets the upgrade to sample the shapes for later use.

slot 2: powence = the mass of bones jumps up in a wave like motion onto a oncomeing army and draining health from them as well as increasein his size faster than if he where to increase in size normally

Slot 3:split = the mass of bones splits itself into two causeing the health to be split between the two forms

Slot 4: Change shape to giant skelton

In his scecound form he changes into a giant skelton the size of which dependes on how many skelotns he has exsorbed so far.

slot 1: slash = the giant skelton form reaches down and slashes the ground

slot 2: dark beam = the giant skelton opens its mouth and lets out a giant scream and a giant dark energy beam comes out

slot 3: dark radiel attack = the giant skelton lets out a giant scream and raises its arms upward as dark energy and bones start shooting out of the gound in a circle around him.

slot 4: change form to form 1

this is a just a idea of how his skill tree would go

mimic and if gets max mimic and split he can learn absorb which will allow him to sample the shape for latter use

split

poerce

slash

dark beam

dark readiel attack

this is just my idea of what a good shape shifting char would be