Spoiler
Show
Shar was the first. She existed for an unfathomable period in the time before time with absolutely nothing else; if the Naught That Was before time or space came to be had a name, it would’ve been Shar. And she was content. But then the idea of "herself" occured to her, along with the name Shar. She'd become aware. But this awareness of self implied the possibility of "something that was not self". As soon as Shar thought that, it spontaneously started to exist. Which was happening inside Shar on account of there being nothing else for it to exist in.
This other being bothered Shar immensely, she didn't want something else tainting her newfound self. So she strained, exerted, pushed and labored until she finally expelled the impurity from inside herself, along with space and time so this other thing could be outside of Shar. Thus the universe was given birth after Shar had impregnated herself with the idea of self and then expelled her twin from her insides. Overmother Selune came to be along with the universe and she immediately became fond of it.
Shar, however, hated it. She was omnipotent, omnipresent and omniscient, accustomed to being the only thing after an infinite period of solitude. The existence of anything, even empty space without motes of dust in it, even the unnoticeable passage of time, was like a maddening cacophony to Shar's infinitely acute and infinitely many senses. She started thinking about how to erase the universe along with Selune. Luminous Overmother, who was also omnipotent, omnipresent and omniscient, immediately heard Shar's thoughts. So they fought.
Overmothers’ unimaginable fury soon coalesced into the Elder Elemental Evil, which would later be called Tharizdun. The First Son was made of pure malevolence and almost comparable in power to his mothers, he promptly attacked them both. Their three way battle lasted an unknown amount of time and Tharizdun was losing, if ever so slowly. So he solidified his divine power, creating the first elements (and by extension everything material). He had constrained himself to a smaller part of the infinite space that was the universe but his essence was much denser now, Tharizdun’s divine power per unit volume exceeded Overmothers’ divine power per unit volume inside Elemental Chaos. While this made him indestructable even to Overmothers, it allowed them to trap him inside his own solidified power and then amputate that part of universe and divide it up, creating five other universes that came to be known as Elemental Planes and the Elemental Chaos. Tharizdun himself had been stopped but now a multiverse existed, which was even more offensive to Shar, and the two twins immediately went back to fighting.
But now they were being cautious. They didn’t want to create another Tharizdun, so Overmothers never attacked with their full strength ever again. But even while pulling her punches, Shar had infinite fertility. Their fighting spawned a number of very powerful beings (who’d all become greater gods leading pantheons of their lessers in the future). Shar impregnated herself with her own anger, giving birth to new and terrible universes that would later become the lower planes. Her attempts to trap Selune as they had done to Tharizdun created even more universes, her omniscience telling her these would become the upper planes after order was brought to multiverse. Even sundering time to break the chains of paradox her omniscience created (which guaranteed her inability to win) only caused the creation of fate, trapping Shar and the entire multiverse inside a mobile infinitesimal speck of time that moves only forward called the present, preventing her from moving back and forth along time to influence things as she wished. The harder Shar fought, the more things started to exist. The more things that existed, the madder she got.
At some point, even the Allmother’s divine mind snapped. She fell to insanity, gave up fighting and curled into a cosmically sobbing ball of despair. Selune took pity on her mothersister and tried to console her, which was the last straw. Shar turned her omnipotence against herself and became the mother of all explosions (she can’t help but be mother of things), a wild and uncontrollable tide of oblivion exploding forth from her form. In an effort to prevent this from destroying most of the multiverse, Selune metaphorically threw herself on the grenade. This halted Shar's power from expanding and consuming most things, ironically creating another couple of planes beyond all the others (Positive and Negative Energy Planes). Shar has never regained control over most of her own self again but is still in there, trapped inside her own ruinous power held in stasis by Selune's light (who needs to devote almost all of her own self to protect the planes from the Maw of Nothing). Shar is still omniscient, knowing every event that happens, sensing everything that can be sensed about everything, hearing every single thought and feeling from all beings everywhere, remembering absolutely everything that’s gone down since day one. She’s been consumed by an incomprehensible fury and madness and desires everything to cease to be.
Today, the greatly reduced Shar is said to dwell somewhere in Negative Energy Plane and is the reason nobody with the slightest bit of self preservation instinct goes there (actually she’s the whole plane but even most gods aren't aware of it). While she’s either completely unknown or just an obscure myth for most beings of the planes, even knowing that Shar really exists is dangerous, for even a simple thought about her is enough to weaken the protection Selune constantly provides to every being in the multiverse and opens one up to the Allmother’s malevolent influence when in Negative. Calling to her in Negative or trying to deliberately draw her attention by worship outside can result in a horrifying dreamvision and forced conversion to a nihilistic and self destructive mentality, followed by induction into her church. It’s possible to find some beings willing to do this because Shar offers something pretty unique, namely an end. It’s incredibly hard to completely annihilate a soul in the multiverse due to Luminous Overmother’s influence, the very few beings who’re able to do so are usually unwilling to destroy a mortal fully, even death or undeath are simply different states of existence. And there’s always some souls who’ve seen too much and would rather not exist at all than continue on with whatever is in their past. To such sinners, repentants, sufferers, nutjobs and fiends, Shar offers several things, either removal of all their painful memories, a chance at revenge for all who wronged them or the merciful embrace of oblivion.
But she's very demanding, unless the worshipper serves loyally and does everything in their power to further her cause, they're gonna be stuck in this existence with all those horrible things that happened. She's not a mere goddess of destruction or death either, the mere acts of killing or destroying doesn't sate her. She demands acts of erasure and despair. Destruction of records and memorials so the past is lost, wholesale destruction so nothing will be left to indicate anything was ever there, genocidal slaughters so nobody will be left alive to even remember the victims, inflicting terrible pain before and after death so souls will be broken enough to beg Shar for oblivion (the easiest way for Shar to bypass Selune’s protection), acts of unspeakable evil to drive victims to despair and desperation. Shar worshippers are almost always beings who figure they're already sunk as low as possible, that things can't possibly be any worse and there's no way to go but up (this is actually true in case of some fiendish outsiders). But nobody gets to become a Shar worshipper without her twisting their mind first. Such beings think nothing of committing the atrocities Shar demands, for it doesn’t matter and they won’t remember their sins once they’re rewarded.
There is a shred of mercy Shar shows to those who're truly suffering in powerlessness with no hope at all. She gives oblivion to those sorts of beings if they beg for it without forcing them into her service, seeing how they couldn't possibly be of use to her anyway. Anyone can pray to Shar for an end and if they'd be completely useless to Shar in any worldly capacity, Shar will instantly kill them and annihilate their soul. This can happen anywhere in the multiverse and is called Allmother's Embrace.
The “worship” of Shar has another, entirely different aspect. All sorts of fertility rituals and prayers for inspiration and creativity invoking her name always succeed, thanks to Shar's uncontrolled omnifertility. Since Shar usually takes this sort of thing as a challenge, terrible stuff is likely to happen to people who do that. Some of the most famous and touching works of art, especially tragedies, were penned by desperate artists who soon became protagonists of stories worthy of Poe, King or even Lovecraft. The farmer with the starving family desperate enough to invoke Shar’s name for bountiful harvests whose fields grew fabulously fertile and attracted ogre marauders who forced him to eat his own slaughtered family is another famous myth that is (in all likelyhood) true. Invoking Shar never ends well.