Crypts don't offer a lot of color possibilities, I think, so I'd try to take an approach using texture. Replacing bricks with plates, and then some of the plates with 1x1 round plates or empty space gives a "broken stone" feel.
Obviously, crypts also need coffins. You can use a 1x2 plate with a 1x2 tile on top for a closed one, or a 1x1 tile and 1x1 plate and put a slightly angled 1x2 tile on top to give the impression it's been opened.
You can try making a few "entrances" as a stone roof supported on pillars. Again, there are some 1x1 round plates to give a hint of a carved relief along the top.
Another common bit of crypt imagery is a catacomb with endless recesses carved into the wall for bodies. A 2x1 plate sandwiched between two 2x2 plates gives a similar effect, and there are several options for what to put in the blank space - or just leave it empty.
For the end I'd imagine a fancy raised altar where the necromancer / lich / mummy / etc. is. Some bright colors here will really stand out even if the build is simple.
For a graveyard outside the crypt, I'd use a combination of the same arches, some simple gravestones, and some of the tall, thin trees from Fortaan. Using some 1x2s with 1 stud you can get a good gravestone layout.