Mfcttrf

This question is for 5e; I've posted a similar question for Pathfinder here.

I'm designing a character for a campaign whose role is to be a recurring antagonist. When the characters first meet him, I want him to seem undeniably human. The idea is that the party will "kill" him and his body will be destroyed, only for him to be revived later and come back for revenge as Lich.

The party will be able to check him for magical effects when they first meet and I want it to be a surprise that he is a Lich. If they party detects Transmutation or Illusion magic on him or detect him as Undead it will give too much of a hint to his true nature. Disguise self could work, but the Lich's face will be seen by the characters and would require a massive skill to pull off.

Is there a way the Lich could retain a human form without magic?

The Lich can instead prevent others from detecting magic on it.

With use of the spells Nondetection or Mindblank, which guard against divination magic, the Lich can prevent a divination spell like Detect Magic from revealing that the Lich is also under the effect of a spell such as Alter Self or Disguise Self. Adding Nystul's Magic Aura into the mix will also protect the Lich from effects like a Paladin's Divine Sense that might reveal that it is an undead. Because Nondetection, Mindblank, and Nystul's Magic Aura are all non-concentration spells, the Lich could have all of them cast on himself in addition to Alter Self or Disguise Self, making him appear human to both physical and magical senses.

Fabricate a flesh suit.

If the goal is just for him not to look like a walking skeleton, he could make a disguise using fabricate and a fresh corpse.

Fabricate won't transmute creatures, but the corpse is dead, and the lich is not actually modifying himself; he can put the mask on his face with glue or something. This will look wrong to anyone who watches him speak or otherwise move his face, so he'll be disguising himself as someone who's been badly disfigured. He'll need a cover story for that.

(Personally, I'd have him spread the rumor that he has leprosy or something. This will both explain his disfigurement and deter anyone from taking a really close look at him.)

One of the reasons I like this is that it gives him a disturbing behavior pattern: the suits don't stay fresh, so every time he wants to meet with you, he has to make a new one, and one of the castle servants (or some other convenient victim) mysteriously disappears and is never seen again, and the other servants will awkwardly deny that such a person ever existed. If you're paying attention you might notice that the Master's eyes are a different color this time.

(I'm assuming fabricate because the reason for avoiding magic is to not have active, detectable spells on himself when he's meeting guests, and in that respect, fabricate is no different from any other way of making the costume. He could use a craft knife and a hot glue gun if he wants, but it's harder.)

Your desired effect is for the party to be surprised that this NPC is a liche. Your desired means is to pull this off without magic. I'll provide the no-magic answer first, and then follow up with how you can achieve the desired effect with magic.

Without Any Magic

Without magic, we are left with the same mundane options available to you or I here in the real world. Heavy application of makeup might do the trick, which is supported by the Disguise Kit item, a tool that a character could be proficient in per the PHB. The Charlatan background includes a skill for creating a false identity through non-magical means - of course, this is an NPC, so there's no need to constrain yourself with PC building mechanics. Xanathar's Guide provides further rules for the creation of nonmagical disguises:

Create Disguise. As part of a long rest, you can create a disguise. It takes you 1 minute to don such a disguise once you have created it. You can carry only one such disguise on you at a time without drawing undue attention, unless you have a bag of holding or a similar method to keep them hidden. Each disguise weighs 1 pound.
At other times, it takes 10 minutes to craft a disguise that involves moderate changes to your appearance, and 30 minutes for one that requires more extensive changes.

Without the use of Gentle Repose, liches are (probably) in a state of constant decay. The application of a strong perfume could both serve to disguise your NPC and serve as a subtle foreshadowing - your players may later reflect "Oh THAT'S why the strong cologne!"

With Magic, But No Effect

Spells and magical effects that physically alter the NPC's appearance, without leaving an ongoing effect, might not be detectable by Detect Magic. Note that True Polymorph, after one hour, is "permanent", no mention of "until dispelled". This suggests that the spell completely changes a creature, and that the effect is not sustained by magic and therefor not detectable but spells that divine the presence of magic.

Disguise Magical Effect

user48255 has more information on this.
Nystul's Magic Aura's mask ability can disguise the lich's creature type. If asked why the NPC has any magical effect at all, they can explain that it's an unfortunate curse, a divine blessing, a spell of protection, etc - you want to give your PCs some kind of hint or foreshadowing, right?