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Keywords are:  'thief skill shadow asylum'

Skill: shadow asylum            Mana:  75  Move:  60  Shadow Cost:  85  Lvl 35 Thie  (21%)
                                (important)   Requires: shadow walk
                                Group: Thief shadow disciplines

Usage:
shadow asylum

The most adept thieves are able to blur the distinction between the physical
world and the shadow realm, and take refuge in a small pocket of the shadow
plane. The shadow asylum persists for as long as the thief remains inside, and
vanishes when the thief exits, leaving him at the same location he entered
from.