Shar and Selûne's relationship pre-Chauntea. Most priests describe them as twins, siblings, sisters, etc., however, they were apparently so close that they have been referred to as the "Two-faced goddess" and similar titles. They could also be read as lovers, since they did have two "daughters" together, though this was far before the concept of family even existed. Hell, Mystryl was created when they, ahem, mixed their essences.
Shar herself. Is she an actual God of Evil, or is she a loving sister and/or lover who felt betrayed when Selûne and Chauntea went behind her back?
Mystra. God all mighty, Mystra. There are two schools of thought on her; those who believe that she is the Realms, and those that believe she is everything wrong with The Realms. One of the most popular questions asked post 4e is when will she come back? The two most popular answers are "In my game she never died " and "Hopefully never. " Interestingly, both sides generally agree that bringing her back would be a bad thing; those who were against her death have either given up on the setting already or treat it as discontinuity in their home games, and those who are for her death will have similar reactions to her being brought back.
Broken Base: Those who like the 4e Forgotten Realms as opposed to those who don't. Or vice versa depending on your point of view. In terms of sales, the group that dislikes the 4e version are "winning", if one can be said to win such arguments. In 2012, Wizards of the Coast announced that Ed Greenwood, the original creator of the setting, has been given the task of "revising" it for the upcoming 5th Edition, and its already been leaked that Greenwood's first job is to repair the damage done by the 4th Edition. The result of 5e is that all the gods that were killed in 3E and 4E, and most of the lands that were destroyed and altered, have been restored... more or less.
Evil Is Sexy: Asmodeus is described as being the most handsome of all devils.
Beshaba also counts.
Averted with Lolth part of the time. In the retcon of her background developed for the Forgotten Realms, her original form after being cast down by Corellon was that of a bloated, hideous spider with an elf's head. But she frequently uses shape-shifting to invoke the trope.
Fandom Rivalry: With fans of Greyhawk in earlier years, then with those of Eberron. Technically goes beyond fandom, as Eberron creator Keith Baker has been known to cite both Forgotten Realms and Dragonlance as examples of everything he set out to subvert.
To go a step further, Forgotten Realms gets so much more attention than all the other major Settings combined, that hating it tends to be a popular pastime for fans of all the other Settings.
Fanon: Discussions and interpretations taking place on the Forgotten Realms sections of the Official Wizards of the Coast D&D Messageboards
Fanon Discontinuity: Many fans choose to ignore the developments in 4E, mostly on the grounds that the 4E setting is less an update and more a new setting. 4E arbitrarily replaced or laid waste to large stretches of the world-as-it-was, used a Time Skip to get rid of most of the mortal cast of characters, and killed off a large number of prominent gods, including Helm, Mystra, Elistraee and almost the entire drow pantheon. This has somewhat changed with 5E, because all the gods that were killed in 3E and 4E and most of the lands that were destroyed and altered have been restored, which led to the return of a number of fans... and just as prominently a flight of others, as some of the changes made in the name of restoring the setting were even more controversial, like destroying the orcish Kingdom of Many-Arrows and forcing orcs back into Always Chaotic Evil status.
Foe Yay: Shar and Selûne were originally so close that they considered each other part of the same whole, often called the Two-Faced Goddess. (Not) Helped by the fact that they have two daughters they created together, Chauntea and Mystryl (though the latter was unintentional).
Game-Breaker: Some consider various high-level NPCs to be Game Breakers.
Asmodeus, the Lord of the Nine Hells, is the Wicked Cultured, Affably Evil, handsome and brilliant Archdevil who, ever since his fall from heaven, has been running a grand chess game that few can even begin to understand. Asmodeus knows every plot against him by the other archdevils and delights in thwarting or subverting them to his own benefit while making his own plots that can take centuries or even millennia to come to fruition in ways that benefit him. At one point, Asmodeus was even deposed from his throne in Nessus, only to later reveal it was simply a ruse to root out potential traitors. Since becoming a god, Asmodeus has only increased his power and effectiveness with some fearing he is perhaps the greatest threat to all the forces of good in existence.
Bane, god of Strife, Tyranny and Hatred was once a mortal man who achieved godhood. A brilliant Chessmaster and strategist who frequently plays other gods against one another, forming and betraying alliances as proper, Bane even leaves a way to return post-mortem after his death after the Time of Troubles, being reborn from the body of his own son. Learning from his defeat, Bane promotes understanding and cooperation among his clergy who are now likely to settle conflict with reasonable debate to stay united against their enemies. Bane shows himself to be as brilliant and dangerous as ever, even subjugating the goblin pantheon to him, stopping at nothing to dominate all other powers before him.
Jarlaxle Baenre, third son of Yvonnel Baenre, survived his birth and sacrifice against all odds, growing into a talented mercenary leader who thrives off the chaos of Menzoberranzan. Constantly hiring out his forces, the Bregan D'Aerthe while playing multiple sides in any conflict, Jarlaxle later goes to the surface, manages to win over nearly everyone he comes across and even uses an attempted coup from his underlings as a way to buy himself a vacation, eliminating the worst traitors and leaving the one Drow he can trust in charge of the group before manipulating powerful kingdoms, ancient dragons and monsters into doing what benefits him most. Jarlaxle is so successful and talented at his games that there are those who wonder if he is actually blessed by the normally virulently misandrist Lolth as a chosen of chaos.
Gromph Baenre is the archmage of Menzoberranzan and a genius manipulator who is the undisputed master of the city's wizards. Keeping control through his own power and constantly playing his rivals against one another, Gromph showcases his penchant for manipulation by being one of the powers behind his sister Triel's throne and when the Priestesses of Lolth lose their power with the Goddess's absence, Gromph takes it upon himself to guide the city in the meantime. After being defeated by the evil lich Dyyr, Gromph is captured by a mind flayer and shows he prepared for a possibility with an impressive Memory Gambit that even programmed his own reactions to achieve it. He then attempts to eliminate one of the architects of the invasion of his city by fusing his hand to a crystal that burns like the sun when the other drow slips into the plane of shadow, trapping him there. Mixing an odd nobility with a ruthless ambition, Gromph repeatedly demonstrates just why he is one of the oldest and most powerful Drow alive.
Mask is the god of thieves and shadows. A master of intrigue and scheming, Mask orchestrated the death of the god Bhaal during the time of Troubles with himself as the weapon in his guise as the sword godsbane. Working to undermine the mad god Cyric, Mask helped to arrange Cyric's defeat and later works to defeat his mother Shar, the goddess of nothingness and darkness. Mask helps to foil her plots to reduce all to nothingness while leaving his essence behind in anticipation for when Shar absorbs him, even sending his Chosen Erevis Cale's son into the future with a plan to bring about his own revival in the end.
Vhaeraun, the Masked Lord, is one of the Dark Seldarine, the Drow pantheon. Wanting to conquer the surface and defeat his mother Lolth, Vhaeraun appeals to many male drow with his focus on gender equality, as opposed to his mother's fierce misandry. An exceptional schemer, Vhaeraun nearly destroyed Lolth during the Silence and upon her rebirth formed a plan to assassinate his chaotic good twin sister Eilistraee, gain her followers and destroy Lolth. Stopped by the goddess Mystra, Vhaeraun later returned and formed a peace with his sister, the two working together against their mother with Vhaeraun organizing his clergy into doing good deeds and appearing respectable, all to work to their true goals of domination and freedom for the drow, no matter the cost.
Fzoul Chembryl, chosen of Bane, joined the Zhentarim and steadily converted it to being a branch of Bane's church. Overthrowing and destroying his mentor Manshoon and many of Manshoon's clones, Fzoul later even helped to undermine Cyric after Bane's fall. Upon Bane's return, Fzoul happily takes his place at his god's side again, reorganizing the church and devoting it to conquest and schemes to advance Bane's aims upon the world. Even after dying in defense of Zhentil Keep, Fzoul is raised as Bane's Exarch, dedicated both to his own power while retaining his charisma and faith.
Hunter's Blades: King Obould Many-Arrows is a brutal orc chieftain who united the fractured orc clans into a single, unified army and unleashed them upon the north and Silverymoon. In a series of brilliant campaigns, Obould carved out the Kingdom of Dark Arrows, becoming the personal chosen of the orc god Gruumsh in the process. Obould also completely subverted a group of drow who thought they could use him, killing their priestess at the end after she attempted to murder him. After assisting the heroes against another army loyal to Gruumsh's old ways, Obould then sued for peace from a position of strength, knowing it will be too costly to dislodge his people from their new home. Peacefully dying later in his sleep and being made an exarch by his god, Obould is without doubt the most successful and ingenious arc to set foot on Faerun and remains one of Drizzt Do'Urden's most deadly foes.
War of the Spider Queen:
Pharaun Mizzrym is the son of House Mizzrym who became a powerful wizard. Well known for his ostentatious, clever manner, Pharaun helps to outwit the Illithid Lich leader of a slave revolt and destroys him after scheming and investigating his way to the bottom of the plot. Pharaun proves himself just as capable in helping to defeat a conspiracy in Ched Nassad, outwitting and destroying a powerful demon later on and even seeing through the machinations of his own ostensible leader Quenthel Baenre. Upon revival from his previous death, Pharaun even begins bonding with his son Kael and shows no hesitation in sacrificing his existence for him at the end.
Nimor Imphraezl, a drow-Shadow Dragon hybrid, and Anointed Blade of the Vhaeraunite Jaezred Chaulssin, is a ruthless assassin out to overthrow the order of Lolth. Manipulating the Menzoberranzan slave revolt, Nimor forms an army to invade Menzoberranzan and throws the city into chaos by assassinating a high-ranking Matron Mother. With his clever strategies and diversions, Nimor nearly arranges for the city to fall before Lolth's return. Later establishing his own fosterage in Menzoberranzan, Nimor continues his schemes to bring down Lolth's order by weaving clever plan after plan and achieving whatever victory he can manage whenever he can.
Roddy's morals were always certainly ambiguous, but his villainy isn't clearly established until he strangles Kellindil with his bare hands.
When Cyric slashed the tendons in a fleeing soldier's ankles and when he ruthlessly killed Midnight's halfling friend. The first could be forgiven by simply saying he had to do it to keep his cover, but the second is unforgivable.
Jarlaxle manipulated events to start a war over the port of Luskan. After the fighting was over, with tons of citizens dead and the city partially destroyed, he made sure that food was scarce by preventing the flow of supplies into the city, and starved the people of Luskan until they were ready to rebel against the new establishment and install Jarlaxle's associate as the next ruler. As Affably Evil as he may be, it's difficult to forgive him for this. The fact that one of the people whose death he causes is Deudermont does not make it any easier. The death toll from the initial war was stated at 4000, with hundreds more dying from the elements after the fact. It's explicit that a good portion of these deaths are civilians and children caught in the crossfire.
Shar arguably crossed it when killing Mystra, who's by technicality her daughter, and one of the most important deities.
Each member of The Dead Three have crossed it at some point.
Bane used his own son (who was evil, but still) as a Soul Jar to ressurrect himself. If he's willing to do that, what isn't he willing to do in order to save himself?
After forseeing his death, Bhaal sired many children (often through rape) to eventually be killed so he can be ressurrected. This insigated the Bhaalspawn Crisis that saw the death of almost every single Bhaalspawn, including the ones who didn't want anything to do with it, as well as countless innocents who got caught in the crossfire.
Myrkul erected the Wall of the Faithless, a wall studded with the souls of Nay-Theists who are slowly and painfully absorbed into the wall. These include people who worshipped false gods, gods who died before they could convert and it's implied to even contain the souls of children. He also turned his rebellious high priest into the Spirit Eater, a Body Surfing curse that is forcing its hosts to eat the souls of anything they come across and will eventually kill the hosts before the curse jumps into another one. He did this to exploit the Gods Need Prayer Badly rule so that people will fear him and he will keep a Not Quite Dead state, should he die. Sensing a pattern here?
The Woobie: Shandril Shessair. When training as a Person of Mass Destruction is the only way that gives some chance to survive... and then she eventually crosses the Despair Event Horizon in Hand of Fire and commits suicide. Convinced that she accidentally fried her true love, Narm Tamaraith, with a spellfire blast. (Hint: She didn't, and Alustriel had to lie her silver-haired hiney off to keep Narm from jumping off a cliff right after her when he finally woke up.) Great way to end the story, Ed. Really.