The Duras Family
Sins of the Father
Duras' sisters, Lursa and B'Etor, attempt to overthrow Gowron by stating Duras had a son and is and should be the rightful heir to the council. They scheme with the Romulans by kidnapping Worf and try to seduce him. When he refuses their advances and proposal of marriage in exchange for privilege, they hand him over to Romulan guards. The Federation is able to outmaneuver them and Gowron keeps his position. But that doesn't stop the sisters, who later scheme with Soren in "Generations" and meet their fate.
Episodes: The Battle & Bloodlines
This little weasel accused Jean-Luc Picard of murdering his son when Picard was captain of The Stargazer. Apparently, Picard's ship fell under attack from a Ferengi vessel in which Bok's son commanded. The crew defended themselves, but that's not how Bok sees it. He hatches a scheme to get back at Picard by using a mind controlling device that not only renders Picard practically useless as captain of The Enterprise because of severe migraines, but has him think he's back on The Stargazer. Picard tries to relive his experience on the salvaged ship and almost destroys The Enterprise in the process using his famous Picard maneuver. Commander Riker is ready for it Picard is outmaneuvered. Bok's scheme is ruined and he's relieved of his command by his comrades.
Bok tries once more to get his revenge, this time using Picard's son as the target. It's revealed that Picard had a love affair with a woman Miranda Vigo and has a son named Jason. The plot is botched when it's learned the "son" had his DNA altered. to make him appear to be Picard's son. The young man is saved in the nick of time. Bok's co-conspirators hand him over when they learn there was no profit to be made.
Episodes: Coming of Age & Conspiracy
Commander Remmick merely starts off as just a pain in the ass to the Enterprise crew in "Coming of Age." He was ordered by Ambassador Quinn to interrogate the senior staff regarding a possible conspiracy to undermine the Federation. The crew comes to dislike Remmick and his accusations. However, Remmick, having found nothing suspicious, tells Picard he found a sense of family and comradery and would like to serve on his ship one day.
Fast forward several months and we find Remmick on Earth with Ambassador Quinn and several other high ranking Starfleet officials at Starfleet headquarters. Only this time, there really is a conspiracy and Remmick is the host of an intelligent life form that must use human carriers for transport and colonization. They are slowly using Starfleet officers to carry out this colonization plan and Dr. Crusher is the next target. Commander Riker infiltrates the group by faking a bonding with one of the aliens by having a breathing tail surgically implanted behind the base of his neck. He and Picard, spared in the nick of time from being forced to eat a bowl of live larva, use phasers to eliminate the aliens from the host bodies. Remmick is destroyed in the most gruesome fashion seen in the series.
The Crystalline Entity
Episodes: Datalore & Silicon Avatar
This "giant snowflake" is hungry. It eats any living thing on any planet. Anything in it's path that moves and has cells is food. We are introduced to it by Data's evil twin Lore first. Lore hatches a plan to have a tree beamed out to it in space for it to eat. That way it can attack the ship and eat the crew when the shields were down. Lucky for the tree, it's Lore that gets his ass kicked out into space. The Crystalline giant moves on to seek other food.
A few years later it shows up to suck up a newly formed colony and taking a few inhabitants with it who weren't lucky enough to get to shelter in time. It's eventually destroyed by a vengeful scientist who kills it with a frequency pulse when the rest of the science crew were on the verge of communicating with it.
Episodes: Elementary, Dear Data & Ship in a Bottle
The Professor is the creation, not only of Sir Arthur Conan Doyle, but the holodeck. Commander's Data and LeForge decide to do a little roleplaying on the holodeck. However, having read all of Doyle's novels, Data solves the mystery in 5 minutes and Geordi refuses to play anymore. So, he, Data, and Dr. Polaskie decide to give Data a real challenge. Geordi asks the computer to create "an opponent capable of defeating Data." Fortunately, this Moriarty wasn't capable of defeating the diplomacy of Captain Picard, who convinces the computer generated sociopath to give back control of the ship to him in exchange for a time when their species has the knowledge and technology to give the professor new life outside the confides of the holodeck.
A few years later, Lt. Barclay meets the professor while doing routine maintenance on the ship's holodecks. The professor tricks him and Captain Picard into thinking he's real enough to journey about the ship and interact with it's inhabitants by creating a simulation in the holodeck. The men discover Moriarty's trick and turn the tables. They allow Moriarty to believe he's actually become a new life form. The professor travels outside the confines of the ship and into his own shuttlecraft with the computer generated love of his life. The whole time, he's in his own computer generated program that's being left to continue running---at least until the ship is dismantled, recommissioned, or destroyed. We never learn about the professor's fate after this episode.