Then, second explanation of Shar and Selune both, along with other lore, could occur something like this shortly after the confrontation with Kagha, Rath and Arabella. Basically, the game has one of your party suggest that you should check out the murals, as in this clip (and the discussion!!! Most of all, the party should be discussing these things as if trying to figure them out together... though maybe not necessarily with all the fan theory as if the game is going to spoon feed you the mystery. I mean, I don't want them to spoon feed me the mystery of what's going on or anything. The point and idea is that they could use this moment, which most people are going to go to Kagha at the grove, to teach people about who Shar is, with more detail than the initial encounter in the Dank Crypt as I detail above):

As the party made their way towards the exit to leave the inner sanctum, Wynari stopped them. She ran up to Ryth-Shan and pointed at a plaque and a mural beyond. "Did you notice this?" she said excitedly. "I figured you'd like it if you hadn’t seen it yet."
Ryth-Shan paused and glanced in the direction she indicated. "No," he said. "I hadn't taken the time to read it." Then he examined them more carefully. "'The forest rose with claw and tooth to tear the darkness from its roots,'" he read straight from the plaque. "Druidic orders often fight dark forces, but I do not recognize the events depicted here. The mural depicts a purge, a sacred cleansing of the land on a scale I've never seen before."
"Me either," said Wynari. "So this didn’t happen over the last hundred and thirty years? I guess if you don't know about it, though, it could have occurred at any time." She then pointed at another plaque and mural. "There's another one there, too. And two more over there."
"Can we just go?" asked Vexir, not interested in the slightest.
Ryth-Shan shook his head. "Could be important. We can't rule out anything." Then he went over to the second plaque. "'By claw and tooth, from root to thorn, The Old Oak's grove, to wildlings sworn.'"
"The druids are elders of a circle, I think," said Wynari. "This place is a divine sanctuary."
Ryth-Shan nodded. "Seems so. No wonder the druids here revere it so much. It could be why the goblins and their leaders want to destroy it so badly. It could be that they are out for revenge, if the darkness had to do with them."
“Oh? You think it’s about goblins and their leaders?” said Wynari. “Nah. I think this grove was once a place of power for someone like Shar, or maybe the Shadow Druids of Cloakwood.”
“Really?” asked Kaedyn. “How did you arrive at that conclusion?”
“‘The forest rose with claw and tooth,’” said Wynari, “‘to tear the darkness from its roots.’ Doesn’t that sound like the servants of nature had to come and defeat the darkness before they could settle here? So, I was thinking that a long time ago there was some sort of evil lurking here. Like I said, maybe it was the Shadow Druids of Cloakwood, or maybe it was just Sharites.” Then she paused for a moment to consider. “I actually assumed they were Sharites because there’s another mural over there that suggests Sharites were involved, at least.”
“But, anyway,” she continued, “I think the first druids came here and purged the grove. Then they established it as a place of power for Silvanus. Then, based on the second mural, I think that Silvanus granted the druids sanctuary here and made them the caretakers as their reward.”
“The darkness could mean anything,” said Ryth-Shan, now deep in thought. “Could be goblins, drow, Sharites, shadow druids, you name it. Either way, I think it’s connected to us.”
"Wait," said Wyll, confused. "You think that the gobbos attacking, the problems with the grove here, and the brain bugs are all a part of some grand scheme of some kind? They're all connected? Am I missing something? How does that make any sense?"
Kaedyn met his gaze. "We've found a number of potential clues. Bhaal, Bane and Myrkul might somehow be a part of this, along with Jergal, Shar, Selune... who knows? We are finding a lot of coincidences and connections.”
“Right,” said Ryth-Shan. “Coincidence? Selune was worshipped heavily in this area. She has a ruined temple in the west. The goblins have taken that temple as their camp. Those same goblins carry shields with a symbol that looks like the blending of the three symbols of Bhaal, Bane and Myrkul. We've recently discovered that the dead drow in the chamber with Nettie had a tadpole in his head, and he was leading some of those same goblins."
"And there's an old crypt of Jergal to the south of here," Gale added. "There, we found a book that is currently, magically recording which gods were dead and have now been made alive by the Second Sundering. There's even been hints of Shar's involvement in the area, and Harpers too."
"And now this," said Ryth-Shan.
"Wait!" said Wynari. "Yes! The other murals indicate Shar and Harpers too. Come on. Take a look." She led them over to the next one. "'In darkest hour, a concord made, twixt harp and wild against the shade.' Harp and wild? Do you recall the old stories of an alliance between druids and the Harpers?"
"A bit, but the details are vague," admitted Ryth-Shan.
Gale took a proud stance, for he, it seemed, knew all about this topic. "Of what alliance are you referring to?" he asked with an air of superiority. "As one really delves into the history of the Harpers, one realizes that they were, in fact, founded by rangers and druids back in the Year of Freedom's Friends, 324 DR - Dale Reckoning, that is. Myth Drannan elven military leaders joined forces with these few trusted human nature-lovers and formed the organization. At the head was Elminster Aumar, renowned mage and, frankly, a friend of mine, actually. He's like a mentor to me. He currently resides in Waterdeep, where I'm from, and he is both aiding the newly appointed Open Lord of Waterdeep, Laeral Silverhand, and he is uncovering a string of murders involving some sort of Masked Lords. I was hoping to help him, but... well... I became caught up in my own personal affairs. And then, all this happened."
He waved that aside. "Anyway, as it stands, druids have a long history with the Harpers," Gale continued. "For example, on the twenty-seventh of Flamerule of 720 DR, at a druid grove in High Dale, known as the Dancing Place, a large congregation of dryads bid the druids to make welcome the priests of many different gods who started to arrive before finally Elminster appeared to explain why they had all been called. This was the First Reformation. They were all called to help fight back against the faithful of Bane, Bhaal, Loviatar, Malar and Myrkul.”
“Again,” said Gale, pausing from his dissertation, “here we bring up the Dead Three. Yes? And, I might add, that the priests who came were priests of Corellon, Mielikki, Mystra, Oghma, Selune, Silvanus, and Tymora. They possessed their own followers during that council to speak through them so that they could make it plain they all needed to work together. That night was known as the Gathering of the Gods."
Vexir was, for once, intrigued. "I suppose you're right. Again, it does seem like we are at the heart of some sort of new God-War. Many of those same gods seem connected to what we are going through."
"So, I wonder if the mural is referring to that event; the Gathering of the Gods," said Kaedyn.
"I doubt it," said Gale. "The murals here seem to imply only an alliance between Harpers and the druids at this grove."
"Are there any other alliances that might fit better?" asked Wyll.
"Let's review the last mural first," said Gale. "It might give better clues."
They moved over to the last mural. Several druids nearby took notice of them, but they seemed totally uninterested. They were too busy arguing over Kagha's decision. Some were pleased that Kagha had released Arabella, and some seemed quite upset by it. Those upset by it expressed their frustration towards the mooching tieflings and their fears that they might not survive the winter because their supplies were being depleted.
Ryth-Shan read off the last plaque's inscription. "'The towers seized, the battle done, the moonrise broke the Darkest One.' Look!" He pointed to the mural. "Wynari’s right. The mark of the dark goddess Shar on the broken helmet. This army marched in her name."
"'Moonrise' must be a reference to Shar's divine sister, Selune," said Kaedyn. “So, once again, we have Shar and Selune involved in this area.”
"I wonder if all this has something to do with the battle of Shar and Selune over the streets of Waterdeep," said Gale as he thought aloud. "In the Year of Shadows, 1358 DR, the Time of Troubles, magic went crazy. The gods were forced to walk the Realms in mortal form.”
“That’s two years after… I mean… that’s roughly when I was taken,” said Wynari.
Gale didn’t even seem to hear her. “Selune was already in mortal form as Luna. Long story short, Shar pretended to be Selune and tricked a lot of people. In the end, the truth was discovered, and Selune battled Shar over the streets of Waterdeep. Her light blasted away Shar's darkness while she reminded her of their unceasing battle and the balance they must uphold. Shar vanished, and the avatar of Selune became Luna once more."
"After this encounter, Selune was free to do as she wished. She made new alliances in her unending war against Shar. It could be that, at that time, she came to this grove and made an alliance here," Gale suggested. "That would be roughly around 1360 DR. Other than this, I can't think of anything in the history books that relates here. There's a library in Waterdeep, though, and one in Baldur's Gate as well, that might shed more light on this. I recall these libraries had a plethora of books on Selune."
"All very interesting," said Ryth-Shan. "We'll have to keep these things in mind as we proceed."

Again, it doesn't have to go exactly llke this, but something like this would be good for people who just don't know much about the backstory of who Shar and Selune are. I mean, if they are major goddesses in the story, with their followers and remnants of their followers everywhere, SOME explanation should be given to new players.