"A lot of it is wizardry. In brief.."
"I'm from a different world. In it, there is an energy called magic, that allows people and creatures to do supernatural things.
There is a type of energy there called necromancy. This deals with animation of dead things, and creation of artificial forms to house unbodied spirits. It also deals with decay, and can be used to prevent or accelerate decay of living creatures.
It is not the only kind of energy, and you aren't restricted to only using one kind of energy. Most of what I can do uses non-necromantic energies.
Anyway, there was a great being where I lived that had bound almost all necromancy in the world to itself. It alone could raise creatures or use the power of necromancy, and all beings that existed through necromancy did so according to its will, and were enthralled to its control. This lasted for centuries.
Until, one day, it was destroyed. All the necromantic energy of an entire world was released, and spread out over a wide area around the location of its death. many, many beings were raised as free undead, unbound spirits found themselves with magical bodies, people were imbued with wild necromantic powers, and so forth.
I was a bird, a kind of bird called a crow that was killed only a few days before the event happened. I was curious about another energy user, a wizard, was doing. When I settled down to investigate, and see what he was reading, I was killed by him, because he thought I was a spy of the great necromancer, some creature observing his movements for an attack.
I didn't like that, and I was angry, and my anger bound my spirit to the earth a few days after my death.. which left me still around at the time the great burst of necromantic energies came forth, which empowered me with new intelligence and certain necromantic abilities, and raised me from the dead.
That was a long time ago, and at that time I was still in my original crow body. The one I am currently in is wholly artificial, created by my own powers and is merely a good imitation of flesh. Damage to it still hurts me, though, and if I'm 'slain' again, I'll be freed as a spirit for some time until I can recreate my body. My abilities will also be hampered.
As for the current body, it is immutable over the short term, like a real body, except that my needs and desires have some unconscious level of control over it. I can carry far more items than a normal creature can, including some nonphysical ones, by putting it 'inside' me in some nonphysical space. This is probably because I like to collect things. I also seem to have grown quite a bit larger than I was, probably because most of my companions have been creatures far, far larger than myself, and it makes things easier.
But at the heart of it I'm a nonphysical being of spirit inside a self-powered artificially-created body."