Cultural and Linguistic note: Guest was the pronunciation of "Ghost" in North England as a form of ghest or geist. So for Eira, a "Guest" is a form of Ghost in keeping with the Celtic/Gaelic/Welsh theme of the setting.
"For thirty years I lived in that house. Every night, I saw her by the tree my great grandfather planted. Every night she stared into my soul her fingers pointing, accusing."
Hit Dice: 2+2
Armor Class: 6 
Attacks: Touch (1d4+level drain)
Move: 12 [60']
Special: Fear, Level Drain
Fear - The Guest emits an aura in a thirty foot radius around itself, all within that aura must make a save or take a -2 to all actions until the Guest is banished or otherwise made to leave.
Level Drain - Their touch causes the target to feel weaker, draining them of level if they cannot successfully save against it.
Spirits that cannot rest, cursed by broken oaths, business left undone, or something else. Left in this world and slowly driven mad by it. Guests were never meant to be in the realms of the mortals and every day they do not pass on is yet another day insanity inducing pain. As undead, Guests cannot be harmed by non-magical weapons unless that weapon happens to be made of silver.