We are playing DnD5e in custom world. This is going to be my first time as DM. Currently I'm trying to get my first adventure together. My idea was to trap my players in the city, which is just an illusion made by Very Evil Necromancer. Just on the very end the players are going to dispel the illusion, and see what they have done.
There is however a problem. First, saying "ok, throw for perception" just when entering the city is going to raise suspiciousness among players, which I do not want. Not even speaking that one can throw 20, and see through illusion at the very start. Also, Detect Magic is going to show them a presence of it.
In PHB, Detect Magic is low-level spell, which is sure to be picked up by at least one player. It shows not only if something is magical or not, but also what school of magic it has. So, "I cast detect magic on this wall. What do I know? - It is magical, magic is of illusion school - There is something hidden there! Lets dig it out".
I do not have my own Dungeon Masters Guide, as I had to lend it to someone, and I can not check, if there is any rule, specified in any official material, that could hide the presence of illusion or any similiar magic. Of course, I could just handwave it away "You were way too weak to see through this" but I want to keep away from such techniques, as I find them unfair.
Is there anything in rules, including drafts from Wizards, that would allow me to create this city without fear that players will find out prematurely? Some kind of detection jammer? The players are quite experienced, and oldest of them was already playing when I was trying to figure out the connection between sounds and letters. Characters are low-level however, not higher than 4 levels. There is bard and warlock, and one more character of unknown class by now.
What I exactly want to achieve: the city was destroyed by an army of undead creatures. The buildings are partially destroyed, but more-or-less in the same shape, so walls are where they were - no need to care about character falling through walls. All green pastures are now ash. Five holy temples of some non-good deities are however immune to magic of evil necromancer, and Necromancer wants them out of the way. To achieve this, he covers the city in illusion, so coming-by heroes are convinced, they were given a quest to save virgins from evil cultists. This includes undead shopkeepers, bartenders, and so on. Necromancer himself is way too powerful to deal with, and this is beginning of long campaign, which will ultimately lead to alliance with said necromancer against The Greater Evil From Beyond.