so in a campaign I’m playing in the Dm let slip that the talking skull with us who teaches our wizard spells when we lv isn’t a fey “knowledge spirit” as he asked about my new seasonal change as he had no idea why my Eladrin had shifted colours/seasons. This lead me to demanding if he was a really a fey as he claimed and as more and more came out it was apparent he has lied to the group from the start (lv 1). We tried using command on him to no avail as he wouldn’t willingly submit, detect fey,fiend,undead revealed he was undead, and zone of truth to no avail as he continued to pass every save as well and not willingly submit. It became completely clear at this point he was a demilich to us and we are pretty sure he’s manipulating us to gather artifacts of an ancient lich/god. Having tipped his hand early , how do we deal with this?

We are only lv 5 and exist in a stupidly low magic level world where lv 3 spells is considered hyper rare/powerful and magic items are hard to find/are 90% of the time cursed. We have about 2000 gold amongst us all and the Dm never gives the wizard scrolls. We have a diviner wizard who’s alignment has been forced to evil via unknowingly using the eye of vecna, a war cleric (in game boyfriend, Rl husband) who she used the eye to resurrect, a shepherd Druid whose being infected by Slaad stuff, a Dragonborn battle master and a Draconic sorcerer. Even though we in game have no idea what a demilich is our characters are aware the skull has lied, is undead, and is super knowledgeable about magic, and we believe likely evil. He may even be vecna. 

Our magic items include a spellbook which i believe vecna or a minion of his writes to the wizard in while pretending to be someone else as the skull, an axe that sends our cleric into a frenzy, pipes that cause fear, a ring that I think makes the wielder hallucinate,  pearl of power, and a wand of +1 damage to fire spells on one dice roll. We also know where a crystal ball is but may have to unearth and fight a frost dragon for it. 

How do we as a lv 5 party destroy a  (seems unlikely Cr 18-21) demilich before he starts eating souls and restoring himself and remembering what he is?, contain it safely until we are strong enough (more likely) , or discover more about it without falling into its plans? Also I believe the Dm doesn’t intend to go past 10th lv which makes this seem ... impossible . Yet I suppose strahd was defeated with artifacts. We are in the points of light/Nentir Vale @Harkonwald. 

any help and ideas for brainstorming would be much appreciated. 

See if you can pick up a couple wands of magic missile. Bide your time, use them lightly - 1 charge at a time.

Wait.

Then when the skull gets damaged (assuming it's a demilich) hit it with the full 7 charges from both wands.

If you do Zone of Truth as written, it is unlikely that they will pass every save.  They will probably refuse to answer, but they won't pass the 100 saves.

As for killing it, I do like the the magic missile wands that anyone can use.  Alternately, put on earplugs, toss it in a bag, tie closed, now you just need to worry about avoiding getting your max hp reduced to 0.

You never know it just might be a "friendly" flameskull or something.

First of all, that party sounds like a massive cluster****, and I love it.

Second, trick it into battling something that equals or exceeds it in power. Track down an ancient dragon, preferably a metallic one, and ask it for help.