So I'm playing in a Viking-ish setting. I'm wondering if I can play as a lich? In terms of traits I would use whatever the lich was in life (human) and not aging and immune to disease or hunger/drinking issues. On the downside, in the presence of say, a cleric with a good alignment presenting a holy symbol, the lich would shrink away. Any thoughts? Is a lich illegal? Legal?

I want to be a lich. It would have traits equal to what it had in life (human) and not aging, and immunity to disease and hunger/water issues. Any thoughts? DMs, would you allow characters to be liches? Is it even legal?

That would depend on the DM.  By lore, it takes being a wizard that can cast 9th level spells and a ritual.  Also need to be evil aligned, since part of the ritual requires feeding the souls of newborns or something to the phylactery.  If you want immunity to disease and not needing to eat/drink, play a warforged.

As far as "legality", it all depends on if the DM allows homebrew since there are no official options for starting as a lich.

Is being a lich legal? I want to play as a lich but I'm unsure if I can. Any thoughts? 

btw I would be a Wizard

I'd say no. Go for a school of necromancy wizard. A lich is what you might become if you reach level 20 and undergo some kind of ritual (involving a lot of sacrificing other people to some sort of demonic entity) to become a lich. Liches are crazy powerful.

Okay, what about making a pact/deal/favor with Orcus for example?

ok fair enough, what about a weak lich?

That is something you could potentially do, most likely with you becoming a warlock of the fiend.  At that point it is backstory and story hooks the DM can work with/provide.  Just make sure your DM is cool with the idea.

Well.. OK, from the description of a lich...

"Liches are the remains of great wizards who embrace undeath as a means of preserving themselves. They further their own power at any cost, having no interest in the affairs of the living except where those affairs interfere with their own.  ... No wizard takes up the path to lichdom on a whim, and the process of becoming a lich is a well-guarded secret. Wizards that seek lichdom must make bargains with fiends, evil gods, or other foul entities. ... A lich is created by an arcane ritual that traps the wizard’s soul within a phylactery. Doing so binds the soul to the mortal world, preventing it from traveling to the Outer Planes after death."

So, again, to me - a lich is sort of an epic-level end state. If you take out its high level stuff (lair actions, legendary actions, damage resistances and immunities, etc. etc.) it comes down to the phylactery that prevents a lich from truly dying, and the ritual that creates it. Otherwise, a lich is just a powerful necromantic wizard.

So to make a weak lich I'd still say "play a school of necromancy wizard".

If by "legal" you mean "in the official rules" then no. And any sort of homebrew that would let you play as a Lich would have to be significantly less powerful than the Lich stats from the Monster Manual. Liches are VERY powerful undead beings, and honestly any homebrew that depowers them enough to be a properly balanced player option would be a Lich in name only. Plus, being an adventurer in a party doesn't really fit a Lich's traditional personality. What is it you're trying to accomplish with this character?

Most liches are Wizards due to their spell versatility making it easier for them to find the spells they need. 

However, there is no specifically set rules on how to become one, because even in D&D lore different liches have achieved their lichdom in different ways. The method you need is best discussed between you and the DM.

For one example of lichdom look at the descriptive text for the Lich entry in the Basic Rules or Monster Manuals.

You also made a thread about this in the Wizards forum and I replied to that but since this is more active I'll copy my reply here.

Quote from Cyb3rM1nd >>

Most liches are Wizards due to their spell versatility making it easier for them to find the spells they need. 

However, there is no specifically set rules on how to become one, because even in D&D lore different liches have achieved their lichdom in different ways. The method you need is best discussed between you and the DM.

For one example of lichdom look at the descriptive text for the Lich entry in the Basic Rules or Monster Manuals.

Why not a wight? A Norse draugr is somewhat like a wight.

Ultimately, you don't need our permission, you need your DM's permission.

Personally, I wouldn't allow a Lich as a character race to start with, for the reasons Maestrino pointed out.

Now ... a Character whose ambition is to become a Lich - that's a different kettle of fish. I'd allow that. It doesn't tell you ( or the Player ) "no, you can't even do that", and it gives the Character some guiding goals.

It also give the DM some guidance about what kind of plot elements to include for that Character.

15th level monk is immune to disease and poison, doesn't need to eat or drink, and doesn't visibly age though can die of old age.

PURITY OF BODY
At 10th level, your mastery of the ki flowing through you makes you immune to disease and poison.

TIMELESS BODY
At 15th level, your ki sustains you so that you suffer none of the frailty of old age, and you can't he aged magically. You can still die of old age, however. In addition, you no longer need food or water.

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On the other hand a warforged doesn't seem to have a known lifespan, doesn't need to eat, drink, sleep or breathe. If you want immune to disease and poison, try a warforged monk.

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Liches on the other hand are a different story and it will be up to your DM whether they would allow you to play any sort of undead. Most wouldn't.

there are a few homebrew "classes" knocking around reddit, but I think any DM would likely say it'll be a campaign long character arch to achieve it, your best bet might be to ask your DM if you character can have a quest to find the Book of Vile Darkness (pg 222 of the Dungeons Masters Guide) as that would have the in game knowledge of how to become a lich, just be aware most goodly organizations and adventurers would want to stop you from becoming a lich. There is however a possible good lich option. Doing a little dive into some old d7D books, in the 3.5e of D&D there were good liches, if you can find a copy of "Monsters of Faerun" it'll give you info on Archlichs and Baelnorn (page 90) which are both goodly aligned lichs, sadly it doesn't give you a detailed break down of how to do it so it'll be DM's choice on that one.

Quote from KafueLechwe >>

Okay, what about making a pact/deal/favor with Orcus for example?

 That would be a good example for the undying patron warlock. At level 1 you get advantage against disease and at level 10 it ages slower and doesnt need to breathe, eat, drink, or sleep.

ok, so it looks like I can be a lich, but in name only, and certain classes would grant lich-like things. What about Archliches?

Ok, what about being a lich in name only... and taking like Undying for Warlock or Shadow Magic for sorcerer? Wizard could wing just about anything.

My DM is preetty chill, and as long as it's balanced would probably say yes. So here are my 2 lich traits/ stats

1. Just take human traits.

or

2. +2 to Int. Don't age, immune to disease, and you count as undead. 

Any thoughts?