A Subtle Threat

Here's another Castlevania monster for Halloween.  Sorry this one's a bit late in coming. 

For this monster, I want another threat that is not just a hit point bash then collect the XP.  This monster is a nuisance that will hopefully make for interesting encounters, once the players finally figure out what it's doing to them.

Bitterfly
AC: 3 (17)
HD: 1**
Move: 30 (10), Fly 180 (60)
Attacks: 1 touch
Damage: poison
No. Appearing: 3-12 (3-12)
Save As: Fighter 1
Morale: 8
Treasure Type: nil
Alignment: Chaotic
XP:16

Bitterflies are creatures with skull-shaped bodies with large moth wings sprouting from the side.   They can change their coloration for short times to match their background, surprising on a roll of 1-4 on d6.  Once detected, they may disappear again on a roll of 1 on d6 even while being observed.  They do not usually attack directly, but when they do, their touch is toxic.  Opponents hit must Save vs. Poison or die in 1d4 Turns.  The biggest threat that these creatures pose is an aura of doom that they project.  Creatures within 20' of a bitterfly must Save vs. Spells or be affected as with a blight spell for one hour.  Undead, constructs, elementals, and slimes/oozes are unaffected by this.  The auras of multiple bitterflies are not cumulative in effect, but the area of effect can be increased.