I am planning an encounter in which a cabal of evil undead necromancers fortify inside an important building with some commoner hostages, disguising the commoners in the same clothes as them to dissuade random attacking and force negotiations.
This tactic could easily be defeated by spells such as detect evil or detect undead, so how (if they could) would the necromancers protect against such common spells? What kind of contingencies would they prepare for such tactics?
The Misdirection spell allows one to swap divination answers with a target. This means the Necromancers would now detect as the commoners they cast it on would, while those commoners would now detect as them. It does, however, allow a will save for the divining character to see through the illusion.
The best way I know of would be to cast imbue with aura which is a level 2 cleric spell and last one minuter per caster level. This only gives them the same aura. To make them detect as undead as well you would need to figure out how to cast undead anatomy. This will make them detect as undead but I dont think it actually changes alignment (I could be wrong).
Now, as undead anatomy is a personal spell, you casting it on another creature, doesnt work. So use the answers here.
You could also make some magic item, or as DM, handwavium.
You don't specifically need the hostages to detect as evil undead - you could simply prevent the necromancers from being distinguishable from them.
Nondetection lasts an hour per level and protects against several spells, including "detect" spells. It also has a Communal variation. Note that Lesser Nondetection only lasts a minute per level, probably not long enough for the plan being described.
Alternatively, Undetectable Alignment could protect against alignment detection, while Appearance of Life could protect against Detect Undead. Although this involves more spells, they're lower level or affect more people, so they could be a cheaper alternative.
Whether polymorph effects like Alter Self would disguise undead is a little unclear. They cause you to lose extraordinary and supernatural abilities, but the "undead aura" that Detect Undead supposedly detects isn't listed in stat blocks. I think that makes it up to the GM. Personally, I'd say anything that changes your type makes you no longer detectable as undead.
Finally, the necromancers might be able to disguise themselves in clothing lined with lead, since a thin sheet of lead blocks "detect" spells. There's even a third-party resource for lead-lined materials (it doesn't specifically say in the description of the material that it blocks such spells - that's in the spell descriptions themselves). Of course, the downside of this approach is that your PCs might start wanting their own lead-lined clothing...
Actually turning the people into something that detects as undead is difficult - most polymorph spells can only be cast on yourself or limit what you can turn people into. However, because there are spells that hide auras, you can use other effects to try and fool people.
Spells like Disfiguring Touch or Slough could give people the appearance of undead or monsters.
Polymorph has a couple of limitations. It can't technically turn people into undead, but there are humanoids like fetchlings and dhampirs that many people would probably mistake for undead. It also requires a willing subject. However, since the targets are already hostages, I think a threat of "accept the effects of this spell or I'll kill you" gets around that.
Those spells don’t work like that.
Detect evil just gives you a count of the number and strength of the evil auras in your cone. It does not let you discriminate between different things in your cone. So if you have a bundle of 50 evil arrows, you can duct tape them to your hostages to make budget-conscious evil peasants.
Detect undead is a little harder, because after studying, it gives you the “exact location” of the undead. It’s not clear how exact “exact” is (I would guess exact means “which 5-foot square”), but you can probably stuff zombie rats in everyone’s pants to get hostages that cannot be distinguished from commoner skeletons.
