Old School Runescape doesn't bother with a 'main questline', it's a lot more sandboxy. But the questlines themselves are still pretty well-developed. As an example, the Ardougne/Elf Questline (entirely from memory):
-Plague City: The city of Ardougne has been split into East and West since it was founded, but recently, a plague outbreak in the West city has led to a quarantine. A man named Jethick asks you to help rescue his daughter Elena, helping you sneak into the city via the sewers. Elena was arrested by the Mourners, a group of volunteer plague-healers, for trying to research the plague itself, but you manage to return her to her father.
-Biohazard: Elena's research into the plague has hit a dead end, since most of her research notes and equipment were confiscated when she was arrested, and she couldn't bring them with her. You sneak back into West Ardougne to recover them, at which point Elena asks you to deliver her notes and a sample of the plague to a colleague of hers. When he replicates Elena's experiments, he reveals that the plague, seemingly, doesn't exist at all. Confronting King Lathas of East Ardougne with this discovery, he admits the truth: the plague is a cover-up for something far worse lurking beneath West Ardougne.
-Underground Pass: King Tyras, Lathas's brother and the ruler of West Ardougne, went on an expedition to the lands of the elves to the west of the continent. While there, he uncovered a seal preventing 'The Dark Lord' from entering this plane. Lathas has been planning a military incursion to pursue his brother and bring him to justice, but the only viable route is through an underground pass ruled over by Iban, a powerful evil sorcerer. You're required to travel through the pass yourself to secure a route and defeat Iban by recreating his phylactery.
-Regicide: With a connection finally established, you travel to the land of the elves, meeting Lord Iorwerth, Lathas's ally in the region. Tyras has allied himself with a rebel faction of elves, in turn. With the help of Iorwerth and his clan, you pinpoint the location of Tyras's camp, then use a catapult and an improvised bomb to assassinate him. Upon returning to East Ardougne to report your success to King Lathas, though, a rebel elf named Arianwyn unseals the letter Iorwerth gave you, revealing that Lathas all along has been attempting to free The Dark Lord, and Tyras was trying to stop him.
-Roving Elves: Something of an interlude quest. To earn the trust of the rebel elves, you need to re-consecrate the grave of a former elven queen, Glarial, reuniting her with her husband King Baxtorian in the afterlife.
-Mourning's End, Part 1: Arianwyn reveals that the Mourners in West Ardougne are actually Iorwerth's elves, disguising their features using plague masks and robes. They've been searching for something in the city, and your job is to infiltrate them and find out what. The Mourners' leader, Essyllt, requires you to create more fake plague symptoms around the city in order to earn his trust. Once you do, he reveals that the 'plague victims' in West Ardougne are being kidnapped and forced as slaves to dig a tunnel beneath the city towards the Temple of Light, the seal of the Dark Lord.
-Mourning's End, Part 2: The Temple of Light is discovered, and Essyllt's advance team is slaughtered by its guardians. Seeing an opportunity, Arianwyn gives you a crystal that can be used to reinforce the seal; however, to reach the point in the temple where you need to use it, you'll have to solve the game's longest and most frustrating puzzle. Eventually, you succeed in your task, buying time for the rebels to gain a foothold against Iorwerth and Lathas.
-Song of the Elves:
The quest begins back in Ardougne, where Lathas has arrested Elena for telling people that the plague isn't real; even though she's right, Lathas claims he wants to avoid sparking mass panic in his city. He doesn't know that you know he's working for the Dark Lord at this point. You break her out and she joins the Ardougne resistance. Disguised as a city official, you implement fake unpopular policies like additional taxes to gain recruits for the resistance. Eventually, you storm the Mourners' headquarters and then Lathas's castle, overthrowing him. At his execution, Lathas is unrepentant, claiming that the Dark Lord will take his ungrateful citizens.
As he dies, you and Elena are knocked out by unseen assailants and brought to Iorwerth's camp. Arianwyn appears and asks why Iorwerth is so dedicated to freeing the Dark Lord; Iorwerth reveals that it is actually a fragment of the elves' patron goddess, Seren, who was left behind when the Edict of Guthix cast the gods out of the world. Iorwerth releases you and Elena, but threatens that he won't be so merciful if you interfere with him again. Wanting to know the truth, Arianwyn asks you to investigate Baxtorian's grave once again.
The statue of Baxtorian turns out to have been the elf himself, petrified in stone. He left the elven lands when Seren disappeared, finding a new land to rule over - in the elves' now-fallen city of Prifddinas, he was the leader of one of the eight clans. With his help, you track down the other clan leaders; none of them can confirm Iorwerth's words, but with the help of a special spell, you're sent into Prifddinas (currently sealed inside a crystal) to search the Grand Library for information. Recordings left by Seren herself reveal that her attempts to make the elves immortal came with a side effect: they would lose their longevity if they ever left her side. She attempted to split that part of herself off, only for the part that was split to be her anger and hatred embodied, forcing her to seal it away.
Iorwerth's forces attack the rebels' base, and Arianwyn changes sides, wanting to see if Seren truly can be revived through this fragment. The rebels and the remaining army of King Tyras reinforce the Temple of Light, leading to a climactic battle. While you're fighting Essyllt, Iorwerth and Arianwyn sneak into the Temple in the chaos. You go after them, but are too late to stop Iorwerth from freeing the Fragment of Seren, who mortally wounds him before dragging you into another battle. You destroy the Fragment, and Iorwerth curses you for it; as incomplete and cursed as it was, it was the elves' last, best hope of ever seeing their goddess again.
After the battle, Arianwyn shares one last conversation with you about the Fragment, where you claim it never would have worked as Iorwerth wanted. Arianwyn voluntarily goes into exile, while Baxtorian and the other clan leaders, now that the civil war is over, restore Prifddinas to its original glory.
