Times an antagonist turned good?

[RogueGuy]

I'll go first.

Characters:
Leon Silversword: Human Rogue (Me)
Pervince Heathertoe: Halfling Cleric (Brother)
Elizabeth Shieldmoore: Human Fighter (GF)
Xyrobi Stormpipe: Gnome Barbarian (Aunt)
DM: (Dad)

-While exploring the ruins of a fallen kingdom, we encounter a human woman wearing a beautiful, elaborate dress.

Woman: "Hello, adventurers. What brings you to my kingdom?"

Leon: "Who are you?"

Woman: "Your new master, boy."

DM: "Roll a wisdom saving throw."

Me: "Hang on, I'll use the Amulet of Mind Mirror."

"Amulet of Mind Mirror: Once per day, if an enemy casts an enchantment or illusion spell on you, you may counter it, forcing them to make the save instead of you. Upon failure, they will act as though you had cast the spell on them."

DM: "OK, she needs to roll her save......."

DM: ".....she fails it."

Woman: "What is your wish, my king?"

Leon: "Holy crap, I can't believe that worked."

She stayed with us for the rest of the campaign, obeying my every command.

[MarkVIIIMarc]

That came in handy!

[Specter]

I'll be second: in one campaign, Malakai, the leader of the underworld organization Unterwegs, antagonized the heroic players and even beat the Paladin in a tournament. After all was said and done, he and his trusted rogue lackeys covered for the players against a band of evil monks as they went ahead to face Azimadia, the fey queen who wanted to decimate all mankind. As a parting word, he said, "this victory is yours. Savor it".

He's still alive through a Clone, but players don't know it.

[JNAProductions]

Boss Macragge. Mafia mob ruler of the underground.

Ended up being the only competent person to take charge when the daemons came out to play.

[Dachimotsu]

Once, my players Diplomacy'd a medusa into docility, then agreed to take out her son who transformed her into one.

[MaxWilson]

Players were exploring a haunted castle as a break from the ongoing hobgoblin invasion. Ancient vampire named Edgewalker inhabits it. Players have several interactions with him and avoid antagonizing him every time. This was before Curse of Strahd came out, but it's basically similar to how Strahd is encouraged to interact with the PCs with semi-friendly interest at first--and in this case, the players managed to stay on his good side until, due to another storyline, the players witnessed a Grey Slaad invade the vampire's domain and make a strange threat against the vampire which visibly unnerved him although the players didn't fully understand the import of it. (One of the players was instrumental in escorting the Slaad into the vampire's sanctum in the first place.)

Long story short, the players ended up negotiating an alliance with the vampire on behalf of their king. He'd help them with their hobgoblin problem, and their kingdom would help him with the... whatever it was the Slaads were up to. The king ended up jumping at the offer and the vampire was brought on board as a "good guy."

In the end this alliance ended up destroying the kingdom and indirectly leading to the end of the campaign. Failure mode: after goblinoids took over the whole planet, the player characters fled the planet and lived out the remainder of their days as modestly successful space merchants based on a nearby air world. (Also, Edgewalker died in combat against the players' foes. So did the Slaad in fact, later.)

[Spiritchaser]

A halfling pirate was literally the last man standing after the PCs ransacked the pirates underground island base (sea caves and pirates ftw). He surrendered in order to avoid death, and the PCs decided they rather liked his sense of humor and kept him around. At first he lived by his charm, but now they're starting to trust him a bit and they even helped him disguise his identity when a priestess of justice who knew him entered the picture. He's decided he quite likes having companions who don't threaten to collect his teeth as souvenirs on a bi-weekly basis.

The PCs are currently engaged in a strained alliance with a group of bandits who once served in the legions with the halfling's now dead pirate companions, and no-one has figured out any of the history yet, I don't quite know what is going to happen... It may not end well...

Edit: still going well... Secret came out last night and the players had a bit of a shock, but no hats on the floor.