So I have an undead adventure coming up where the pcs are attacked by undead while making their way through a mountain pass. After a rally step more undead attack, and the enounter repeats until they flee to a nearby keep (that is under the control of the necromancer who is raising all these undead).

Anyway I'm wondering what sort of things would you do to add atmosphere to these "rentless" encounters to increase the spookyness and to convey the idea that they are up against an army of undead?

Also, do you think that shooting against skeletons should somehow be penalized? It seems like crossbow bolts and arrows wouldn't do much against them.

As it's a mountain pass you have the scene of an ancient battle between say Greenskins and humans (and maybe dwarfs) this will give you scope to add lots of different types of skeletons for varying difficulty, eg, stone troll skeletons.

Maybe the necro's base was built on the site of an ancient ruined Elven tower, or over a lost minor dwarf hold, to give spooky ghost and spirit potential.

Carrion birds circling above, just before anything interesting happens.

A route lined with skeletons on poles to warn off the descendants of the vanquished, whichever side that was.

Loads of tattered banners in a consistent colour scheme. And the skeletons marching in step and drilling as if they were a regular army.

Use spooky music. Almost anything from Sleepy hollow soundtarck will do. Especially Tree of the Dead Track has some good ups and downs tempo which can create tension.

another few useful, some of which I used in my recent undead session:

Superfreeze from Day after tomorrow - for when they see the undead armies march or when they flee them

Entering the Stargate from Stargate - for when they enter the pass and first get the feeling of something amiss

Tornado Warning from Day after Tomorrow - good opening

Track 7 from King Kong - good tension and battle (drums)

Music is very effective for creating subconcious feelings in people (your players)