dnd-3.5eforgotten-realmspathfinder-1e
According to the rules,
The process of becoming a lich is unspeakably evil and can be undertaken only by a willing character.
What if a lich, having done this, slowly over time realizes its ways were wrong? Do the rules
The lich template requires that you create a phylactery (see "The Lich's Phylactery" in the d20 SRD entry for Lich).
The phylactery requires Craft Wondrous Items, the ability to cast spells, and a caster level of 11th or higher.
A swordsage cannot cast spells and does not have a caster level, and therefore cannot create the phylactery and cannot become a lich.
Alright, it sounds like you have a lot of problems going on here that I'm going to address individually using the ruleset that you've chosen for your game.
There's no easy way to say this, but your friend is attempting to take advantage of you during the course of this game that you're playing. An item to double someone's Strength score (whether it be armor or a belt) would cost millions of gold pieces and be at least an artifact level item. Under zero circumstances should you ever agree to such a demand within the confines of the rules. He can buy a +6 Belt of Giant Strength just like everyone else.
A magical item has a 20%ish chance of having a magical glow that would indicate that it's magical. It sounds like in this instance your item has this, so a check isn't even necessary. If you want to know its magical properties (and you do not in fact have access to the Wizard spell Identify), your barbarian must roll Spellcraft with a DC of 70 + caster level of the item. If he can't make that check? Tough. He can't identify the properties of the item.
If you want to be nice you can use the following descriptors in my answer on this question to give the players a hint at the items purpose. But that's entirely optional.
With sunder there are steps that you need to follow, that are written so a Barbarian doesn't steal your sword and attempt to snap it over his knee.
You take an attack of opportunity, and you roll an opposed attack roll. If he wins? Deal damage to the sword or shield or armor as per the hardness table for materials in the DMG. If he has Improved Sunder he doesn't incur the AOO. There shouldn't ever be an occurrence where process fails to happen if he's attempting to break a weapon. If you cave on this guy you're showing him that he can bully you, the GM, around, which complicates things.
What he's attempting to do with a door isn't really sunder, and ties into the type of door he's trying to use brute force to break through, and the Break DC of that door. Wooden doors are the easiest to break through, then stone, then iron. All the information about doors, and their break DCs can be found here.
The rules don't say you can't, but the rules don't say a lot of things that common sense would normally take care of. The rules don't say a wizard can't spit lasers or vomit lightning at will, but the rules do say what you can do within the scope of the game. If the rules don't say you can't, it also means they don't say you can also. What information isn't covered by the book can just be fielded by common sense, or house rules.
Bones aren't as strong as iron or stone, or even wood. If he attempts to punch through an obstacle that isn't meant to be broken he breaks his fist, suffering a penalty to all melee attacks until he receives healing of any kind. Making other players useless is awful. It's even more awful if its allowed to happen. So make him useless for attempting to act outside the confines of his role. This will teach him some humility hopefully.
The best way to get rid of this mentality is to punish his kick-in-the-door style of play. There was a module a long time ago published called The Tomb of Horrors in which everything was a trap meant to screw with and punish characters or players with preconceived notions on how the game itself was played. It had things such as Pit traps with poisoned spikes (Save or Die), fake doors that when opened or forced will conjure a spear to stab the closest person, a sphere of annihilation for those who attempt to backtrack to an area after they fell into an area triggered by a prior trap, and secret doors hidden slightly above spikes at the bottom of pit traps.
If you do play something like this module your barbarian friend will trigger every trap, die, and then he can roll up another character, but if you do decide to run something similar to The Tomb of Horrors be sure that everyone you're playing with has at least a minimum of two characters on hand, there will be lots of death involved.
If your barbarian friend is that insistent on trying to brute-force his way through solid objects, implement a series of puzzle doors that require brainpower to solve instead of trying to punch things. It will give your group a chance to converse at length and find a way through the doors using thought and teamwork instead of one guy attempting to brute force through something he has no business trying to break.
Lets say for example your party is in a dungeon, and there's a portcullis in a dungeon. For those of you who don't know a Portcullis is a door normally with bars that falls down instead of opening up. They normally have bars. For this example we'll be using a standard portcullis. On one side of the gate is a large iron wheel which connects to a chain that when turned will raise the Portcullis, However, the wheel is rusted and requires great strength to turn. Once the barbarian turns the wheel, as its pretty much expected for him to do, he will have to attend to the wheel to insure that the Portcullis remains open while the other PCs finish their own challenge.
This is probably your best opportunity to shake the mentality that because he is a player and your friend he gets whatever he wants. Put your foot down. Let him fight the level 40 Demigod with over 1200 hit points who always hits and only has to roll to confirm criticals, with DR 71/+5 and ninety-two strength, to get his belt.
The fight will end in one round and he can roll up another character that might be a little more humble. If you don't want to be this drastic you can do this outside the actual canon of the game to show to him what a ridiculous notion he's attempting to pass.
Just give him a warning. Just warn him that what he's attempting to do doesn't exist or function within the confines of the game rules and if he raises an objection put him in time out for five minutes while you resolve combat for the people who DO want to follow the rules. If he doesn't want to follow the rules, time him out for five minutes. Eventually he'll get the picture that what he's doing isn't all right, but you're going to have to be strict.
What he's doing to you isn't okay. His actions are railroading the game the way HE wants it to be played, not the way you're trying to run it, and that just isn't a cool thing to do at the game table.
While there are valuable tools online for 3.5 related games, such as the Online d20 SRD, Herolabs, etc., sites such as Dandwiki, D&D Tools, and Therafim should be taken as sources with an extreme grain of salt. Oftentimes things on Dandwiki are homebrew content unless specifically marked with the "SRD" descriptor on their title pages. And generally Therafim and D&D Tools (while usually having correct information) host their information illegally: most information you find published on those sites is from books not covered by the OGL.
If he doesn't cite the sources for his character decisions from actual sourcebooks, I would make him find the information before allowing him to even sit down and play at the table, as there's a possibility that he could even be making it up as he goes along to give himself an edge. If he can't find the information about his abilities in an actual sourcebook, have him roll up a new character with the books you have at the table.
Best Answer
Yes...depending
Alignment causes a lot of arguments around the D&D community. There are a pair of rock-solid methods: the Sanctify the Wicked spell, and the redemption rules, both of them found in the Book of Exalted Deeds. The BoED can be a controversial book in some groups, but those rules are a definite method of getting the lich back on the straight and narrow.
And then we get to the less-clear part.
D&D's alignment system can be inconsistent at times. Some sources say alignment is about action; other sources imply that it's about the intent behind an action. There's elements of belief systems and thought patterns in alignment too, which complicates the matter further. The Player's Handbook says alignment is supposed to be a guide; other sources treat it like a straightjacket. All of this makes the question, "Could this lich seek redemption on his own?" extremely difficult to answer.
I am inclined to say yes, no being is past redemption. The idea that any being with free will, down to the most vile and wretched demons, can find redemption is the cornerstone of how the Good alignment treats others. Celestials and fiends are polarized by these opposite beliefs - that any being is capable of redemption (even if it's not) in the case of celestials, and that no being is above corruption (even if it is) in the case of fiends. While an argument could theoretically be made that fiends are past redemption (an argument that's riddled with holes - there are canonical examples of non-evil fiends), the lich you're discussing is a mortal, born, living, and now undead with full control of their own free will. If they have a reason to turn from evil - even if that reason is weariness, or a craving for acceptance - then I don't see a reason that they can't, especially if it makes a good story.
As for true death, there's a few avenues for that - the easiest is for the lich to smash his phylactery, then kill himself. However, there is another option - regaining mortality. Resurrection and True Resurrection can both return an undead being to true life, and by RAW you don't even need to break the phylactery. The lich would be mortal once again, but would be able to live out the rest of their natural years and then die in peace.
As to the greater question of if their act of unspeakable evil to become a lich can be forgiven...in the spiritual sense, the Book of Exalted Deeds states that no being is beyond forgiveness if they truly seek to repent - at least, not in the eyes of the forces of planar Good and the gods of that alignment. Forgiveness from the lich's victims is another matter entirely.
A Note for Pathfinder Games
Pathfinder's alignment system is essentially just 3.5s; however, the developer's attitude towards it tends to be more consistent. Sean K. Reynolds, Jason Buhlman and the various Pathfinder writers gleefully embrace fantasy racism and the idea that a being or being(s) can be born irredeemably evil. That doesn't have to affect how you run your game, but tread lightly in Pathfinder Soceity games.