[Trigger] Metamorphosis

Long time since I posted here, but anyway. I was planning on making a Metamorphosis ability that has a 10% chance when cast to turn the hero into a different hero than it was meant to. Of course my question now is; how do I do this?

I'm not all that fancy with spell triggering so this is a little outta my league. If anyone would be kind enough to answer, please do so in baby steps, I'm still new with variables and stuff like that. Oh, and no JASS, fully GUI, plox.

Can your hero be replaced?

You could add the metamorph buff, (Via aura), and then add a timer, that, when it expires, remove the skill.

So you mean normally you become Demon form(example) but theres a 10% chance you'll become a penguin?(another example)

If so, i guess you could maybe hide the hero and create the new unit, wait and then remove the unit, move hero to where the unit was and unhide hero.

Make an ability based on berserk
make 2 metamorphosises
When someone casts berserk
if random integer between 1 and 100 < 10
Give metamorphosis 2
order him to use that
and remove metamorphosis2
else
do those for 1