The glass egg is your phylactery.
The gas is your one hit point.
If the egg breaks, the gas disappears and you die.
When you are born, the egg pops into existence somewhere near you. It could be the under the bed, or on a shelf in the castle, or in the well in the town square.
If you travel too far from the phylactery, your body starts to decay, accelerating the further you travel from it. But you won't die, as long as the egg is whole.
This is why many folks never move; they haven't found their phylactery, and the fear of travelling too far makes them stay put.
This is also how undead are made; regular people that travel too far from their glass egg, either driven away by force or ignorance. Remove enough flesh and soon you'll have yourself a zombie.
Pushing on even more, you'll end up as a skeleton.
And then, when the last bone withers away, you'll continue as a spectre; a pale sheet of your former self.
Liches are just powerful wizards that forgot to bring their glass egg with them. They stopped travelling when they got tired of flesh falling of their body.