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Lusus naturae ("freaks of nature"), also known as custodians, are the substitutes of parents in Alternian society. They look after the young Trolls while the adults besiege other star systems in the name of Alternian glory.
Trolls and their custodians have a peculiar arrangement of codependency. The lusus behaves as a lifelong bodyguard, caretaker, and mentor, while the young Troll must learn to function as a sort of zookeeper.
They are horrifying beasts found in the ceremonial brooding caverns. Once the young Trolls have completed their many dangerous trials, a lusus will choose a Troll. A Troll's pick of lusus is dependent on their blood colour, as all lusii have the same blood colour as their Trolls. Together the young troll and custodian surface from the subterranean vaults and build a new hive using carpenter droids.
During the game of Sburb, a Trolls lusus will always die. This is because the young players must learn to be independent. It also extremely common for the body of a dead lusus to be one of the items that the kernelsprite is prototyped with. This naturally leads to some pretty horrific enemies, especially down to the lusii of the higher castes.
Chynus' lusus, a Dragon by the name of Arcturios, is the most powerful lusus of the Stemfazed Trolls. Arcturios was killed far prior to the game of Sburb in an alien invasion of Alternia, but Chynus was still able to prototype his kernel sprite with it due to his keeping of two teeth as mementos of his guardian.
Ermira's lusus is like a cross between a tiger and a hedgehog. She is relatively small for a lusus, only around Ermira's own weight, but she is very fast and a good hunter for food. She gives Ermira a lot of freedom, leading in part to her wild personality.
Falvyn's lusus is a giant bunny.
Komsur's lusus is a large, three headed snake by the name of 'Anguilla'. Anguilla is an anglo-saxon word meaning 'snake'. Anguilla is very bloodthirsty, which means Komsur is constantly looking for ways of satiating its hunger. Upon hearing of a similar situation with one of his Flarp associates, he tried luring other Trolls lusii and even the Trolls themselves to its lair. He hated doing this however, so stopped quickly. As a result, his Lusus is often very hungry and the relationship between the two has grown tense. Chynus often hears the hisses of his lusus from underneath Komsur's hive, and it took him a while to adjust to them. Most of the time the three heads of Anguilla can't co-operate together, often making the huge lusus smash into things, causing slight tremors in Komsur's hive.
Athren's lusus is a colossal pink jellyfish by the name of Scyphozoa. It has a very limited power of movement, but it has strong psychic powers allowing it to communicate with Athren as well as lure food into it's reach.
Vriska Serket's lusus is an enormous, monstrous spider like beast. One of it's fangs is bent downwards at the tip like Vriska's horns. It feeds on other lusii and even trolls. Vriska provides "Spidermom" with food through Flarp campaigns, and Komsur was at one point almost eaten by her.
It is worth noting that the concept of a beastly custodian is similar to that of Dæmons of Philip Pullmans 'His Dark Materials' trilogy. Other references to His Dark Materials include Cholerbears, similar to the Panserbjørn.