An intruder in a dream usually reflects boundaries that feel violated -- a sense that your space, privacy, or safety has been compromised somewhere in waking life. The vulnerability in these dreams is often the real message, more than the intruder's identity. If you're able to face or remove the intruder, the dream may be showing you a boundary you're finally ready to defend.
To unfasten the tongues Mercury must be in its house and it is efficient with those who Mercury rules. - Picatrix (Andalusia, ~1000.AD)