The Undead leave nothing behind them
I struggled with current RPG Blog Carnival Prompt for a few reasons. First, it is summer here and it is hot and I am cold-liking creature. Opposite of a lizard. More like pratchettian troll. Second, we're renovating kitchen and bathroom, and it is expensive and not everything can be done in a small flat in post-soviet block and it is stressful. Third, I always struggle.
In my slowly brewing solo playtest PCs faced undead creatures twice. Both of them are Chimeras - creatures made out of twisted together decaying bodies. They are strong, very resilient and don't behave like living beings. Each undead is like a point of unnatural presence, movements too stiff or too erratic, whole world reacts to them.
After the undead attacks you can identify this signs:
Insects are absent from the area. There is no more familiar buzz, less background noise makes creepy atmosphere especially if you suddenly come into the area. Also significant lack of birds in the sky and small animals anywhere near.
Humans living in the area tend to be scared. Some are suspicious of others - those who tend to work with the undead - others are either looking for an occasion to run away or are outright hostile in order to scare away innocent travelers. As a GM if I suspect there may be some undead in the area I automatically give -1 to reaction rolls. They just don't want you here. And probably themselves.
Undead don't have an agenda. They don't plan, they don't go to places because it serves some goal. Or maybe they do. Usually it looks like they go in straight line, follow civilization or some other things - some weirdly follow the sun or the moon and stay in place when cannot find them. Inevitably though they leave after themselves trace of dead bodies. Aside of chimerae. They on the other hand try to incorporate dead or still-living victims into their own tissue, feeding themselves off the local populace.
People are not powerless against the undead. Even though it is very dangerous they usually can be destroyed, captured or trapped. Aside of events when someone smuggles something undead to the city or it is summoned the attacks are happening in the country and in remote places. A crowd can take care of a weaker undead incursion in no time, usually with casualties.
And what happens when someone mad finally finishes off such a creature? Usually nature reclaims the place slowly, undead do not hoard treasures, cannot be used in anything and it is usually just a murderfest left. Sad thing but destroying undead usually leaves societal and civilisational void in the area.