I'm playing a LN paladin (vengeance)/warlock (Great Old One/Tome) soon, and I want to redeem enemies I deem worthy of my mercy.
With create thrall I expect to be in complete control of an enemy at the end of combat.
Is there anything in 5E that would allow me to change someone/thing's alignment? If not, is there anything in previous editions on which I can base such a spell?
There are no RAW ways to change a creature's alignment in 5E as it has become a more "rules-light" element since older editions.
There are however plenty of [Good] spells in 3.5's Book of Exalted Deeds which are related to salvation of evil creatures. One of them that comes into mind is:
Sanctify the Wicked, level 9; traps target's soul in expensive gem, released as good creature after 1 year.
I hope that helps as a baseline, though keep in mind that as others have mentioned, it will require a more RP-orient approach to do something like what you describe without introducing elements/spells/abilities that affect in-game balance and cohesion.
It can be done, but probably not by you. Since you have Create Thrall, you must have at least 14 levels of Warlock, but you'd need 17 levels of Paladin (or 9 levels of Bard, Cleric, Druid, or Wizard) to redeem an evildoer (for 30 days, anyway). 17 levels of Bard, Cleric, Druid, or Wizard to make it permanent.
The spell Geas can be used to command an evildoer to redeem themselves while avoiding having the Geas removed. While this won't technically change their alignment, they will be forced to do good. It should also be pointed out that a sufficiently powerful creature could ignore the damage caused by disobeying the Geas and continue to do evil.
You could try talking to them. Certainly, there are magical options that allow you to forcibly change the alignment of a creature, but the good old fashioned method of explaining to a villain the error of their ways is always an option.
(Admittedly, it's a high-maintenance option that's unlikely to succeed without spending a vast amount of time and effort to get the villain to sit still and think and explain their motivations and their formative childhood experiences and the way they think about ethics, so that you can take that information and use it as the basis for a very persuasive series of discussions of your own worldview and how various "good" ethical and moral stances are superior (both for the individual and the group) even within the villain's own view of the world... But then, if it was easy, they'd just ask mad wizards to quit necromancy, instead of hiring paladins to deal with the problem.)
Regardless of method, redeeming villains through play will only be possible with your GM's consent, and that might be harder to obtain than you suspect; there are a variety of reasons why a GM might want to make villains irredeemable or not focus on redemption as a part of gameplay.
You could make it a roleplay thing. I mean, since you're a warlock AND a paladin I'd assume you have high cha (lay on hands and invocation saving throws) I imagine a few diplomacy throws and a show of mercy could do what you want. granted the DM allows.
This is a similar situation to an issue that I had with a black dragon wyrmling. There is a way to force an alignment change and it would take a simple Geas spell, although I am unsure about the moral justification for it. Forcing someone to be someone they are not is just as evil as anything they are doing.
Anyway Nitsua60 explained this method or rather most of it and we did a little bit of digging and found the Bytopia method.
"There are three ways to forcibly change something's alignment on the books. But none of them really work out well for you....
Forced attunement. Arguably the least dangerous option, if you can somehow force the dragon to attune a magical item you might change its alignment. Perhaps a Geas? If your DM sees that working, and you have any of The Book of Vile Darkness, The Eye of Vecna, or The Hand of Vecna on you, the dragon's alignment will change. To Neutral Evil.
To the Planes. Send your dragon on vacation to the Bytopia for 4 days ("Pervasive Goodwill" optional rule, DMB pp.59-60), The Abyss ("Vile Transformation" optional rule, DMG p.63) or the Nine Hells ("Pervasive Evil" optional rule, DMG p.64) and its alignment might change to Neutral Good, to Neutral Evil, or to Lawful Evil. (But Dispel Magic or Remove Curse will get rid of the Bytopian effect.)
Enlist the Slaadi. Unfortunately (?) this won't work on your dragon--only on humanoids. But in the interests of completeness, here's the third way to forcibly change something's alignment: If you can get a Blue Slaad to bring your (humanoid) target to 0 HP it'll automatically become Chaotic Neutral. Of course that's a consequence of it becoming a Red Slaad."
Simply modify from dragon to "whatever you want"
