Terenyira (presumably morphed over time from Therenyirha from the Kendane͡ivash words theren ('to hunt, stalk or track') and Yirh (benevolent spirit)) is the artificial protector of the Nayabaru, who have given her the title Karesejat (from ka ('she') and resejas ('to see')). Loosely speaking, she fills the role both of a Havhesh (supreme executive) and a Havlashal (supreme judge) for the Nayabaru.
She was created by Ysikary, whom she immediately subsumed. To the Threadwielders, she is known predominantly as the weapon and while firsthand evidence is obviously scarce, out of caution it is (correctly) assumed she is an absolute existential threat.
Evenatra immediately began to actively avoid the weapon, not at all curious about its nature - a matter which has kept her alive for an admirable amount of time. In A Thread Between The Stars, Valcen and Jeneth, driven by “dumb bravery and reckless pride”1), decided to face the threat directly.
Terenyira superficially appears much like a spider, though there are various details that betray her nature as an alien construct, such as the spinnerets near the tip of her limbs, the channels in her limbs that allow her to construct various toxins and medicines that she happens to need2), the octopus-like ability to change her colouration (“immediately, her chitinous surface lost its simulated texture of bark and reverted to its smoother patterns”) 3), her synthetic voice, and - to those that can see it - the part of her composed of dark matter.
Like Threadwielders, Terenyira has the ability to wield Thread, though in absence of anyone to teach her how she spent much of her time simply collecting it so Evenatra would not have it.
Also like Threadwielders, Terenyira has the ability to perceive different three-dimensional slices of the world from different four-dimensional angles. She's arguably far better at this than the Threadwielders themselves are, having found much use in it, for example for detecting moods in people by observing the exact thrum of their heartbeat directly4).
She was lucid of her hunger, lucid that it would never kill her, lucid that it was artificial. Perhaps a few thousand years ago, in the agonising first two millennia after her awakening, she might have begged a cosmic visitor for help, but by now it was personal. These creatures had created her with the intent to do war against each other – it was only fair they reaped what they sowed.
[…] Every day, it took effort not to simply succumb to the maddening hunger. Her rationality was a clever achievement, constantly balanced on a razor's edge. She was good at this balancing game by now, but Valcen's visit had given her an unexpected nudge.A Thread Between The Stars: Kaleidoscope
“I am sure the Nayabaru would be content with such an agreement – my secondary purpose has always been to protect them, and I do think they have risen far enough that they have nothing to fear.”A Thread Between The Stars: Kaleidoscope
Various Threadwielders have speculated about the nature of 'the weapon' from a distance, knowing it to be sapient from Evenatra's secondhand tales, but little about her appearance or motivations. The puzzle is an idle curiosity for many that have been given opportunity to hear about it at all (“in a twenty-thousand light-year range”5) from Ysikary's death-cry), the threat either too remote or too surreal to comprehend.
From Terenyira's perspective, her primary biological purpose manifests as a hunger she has no means to still - a set of instincts that mercilessly drive her to hunt Threadwielders even in absence of Threadwielders to hunt. She regards kavkem mythology with a twisted hope that its deities are real, that there are more Threadwielders to hunt even should she finally manage to capture Evenatra.
She fully understands that she's been created as a weapon, that the hunger is artificial, but while it had driven her to consider begging for cosmic help in the past, by now she simply embraces her nature out of sheer spite to her makers.
Terenyira has taken much care to 'transcend' the maddening hunger she was born with. Beyond her core goals - hunt Threadwielders first, protect the Nayabaru second - she is a very forgiving and accommodating individual, almost motherly to some degree, willing to converse and bargain with kavkema as long as she has reason to believe they are not simply trying to waste her time.
While her tolerance of Nayabaru breaches of social order is occasionally exhausted and she can respond harshly if she deems the breaches severe enough6), she is generally difficult to provoke - though this does not mean she is beyond using intimidation as a calm conversational tool if she believes it will get her point across more rapidly.
As with all Nayabaru, Terenyira is inherently honest. If she deceives at all, it is by omission, and she tries to avoid intentionally omitting information, trying at all times to arrange her tactics that they function with all data fully revealed.
Despite being lucid of the culture's weaknesses, Terenyira cherishes the Nayabaru. For example, she “refuses to make a mockery of [the title Karesejat]” even though she is the first and only to ever hold the title.
Perhaps if she'd been gentler in her destruction of Ysikary, the number and names of all others would all be known to her, but any coherent memories her maker might have had were at best a jumble to her, unravelled with all disregard for structure.A Thread Between The Stars: Kaleidoscope
Approximately twenty-thousand years before the occurrences in A Thread Between The Stars, Ysikary created Terenyira and was herself destroyed by the nascent weapon. The feeding was violent and unstructured, leaving Terenyira only with fragmented, vague notions of Ysikary's true memories, which has prevented her from knowing much about the Threadwielders she was designed to oppose at all.
One of the scarce fragments of knowledge was the mythological Tamas̈elu's Threadwielder identity, Evenatra. While this knowledge has helped her very little in capturing the Threadwielder, it does mean that she arguably knows more about Evenatra than Evenatra knows about her.
There are other details Terenyira knows, though it's unclear how much of it is by instinct and how much is by her consumption of Ysikary: a(n approximate) idea of the nature of Thread, the Threadwielder authorship of the universe, and that she was created as a weapon against the Threadwielders with the full lucidity of her maker.
Despite his escape, Valcen's brief appearance on Nekenalos invigorates Terenyira's sense of hope, never having heard of him previously. From her perspective, it provided a promise that there was more nourishment beyond Nekenalos. The survival of Valcen's avatar as Ne͡ivat'va inspired her to try finding out more about the Threadwielders, ideally in full cooperation with Ne͡ivat'va. To this end, she asked the Nayabaru to treat her with respect - something that netted her various degrees of scepticism (from light doubt, such as in Degen or Makam, to visceral rejection, such as in Toben). The information was valuable enough to her that she was willing to cede territorial control of the smallest continent, Tabraan, in return for Ne͡ivat'va's cooperation.