You go downstairs and confront your custodian, which is another term for a frightening beast known as a LUSUS NATURAE.
Your lusus has looked after you since you were very young in lieu of any biological parents, whom you have never known. No young troll ever knows his or her blood parents, nor could such lineage ever be accurately traced. Adult trolls supply their genetic material to the FILIAL PAILS carried by imperial drones and offered to the monstrous MOTHER GRUB deep underground in the brooding caverns. She then combines all the genetic material into one diabolical incestuous slurry, and lays hundreds of thousands of eggs at once.
The eggs hatch into young larval trolls which wriggle about to locate a cozy stalactite from which to spin their cocoons. After they pupate, the young troll with his or her newfound limbs undergoes a series of dangerous trials. If they survive, they are chosen by a member of the diverse and terrifying subterranean monster population native to Alternia. This creature becomes the troll's lusus, and together they surface and choose a location to build a hive. The building process is facilitated by CARPENTER DROIDS left on the planet to cater to the young. But only for building. They're on their own otherwise.
The vast majority of adult trolls are off-planet, serving some role in the forces of ongoing imperial conquest, besieging other star systems in the name of Alternian glory. The culture and civilization on the homeworld is maintained almost entirely by the young.
Trolls sure are weird!