I've been working on a new type of spirit to use against my players and since I haven't found anything like them on the rest of the net I thought I'd put it up here and hope for comments and critiques. With that said, I give you:

Parasite Spirits:

Along the idea of insect spirits, parasite spirits take hosts, but in different ways and for different ends. The two that I have come up with so far are Leech and Tick (whose domains are stagnant water and land, any but city)

Powers:
1.) Attachment. Through the use of attachment the parasite spirit attaches itself to the Host. Once the spirit is attached it blends it's aura with that of the Host to better conceal itself. This counts as Masking of an Initiate with a rating equal to the force of the spirit. To target the spirit itself the Masking must be penetrated, else only the Host takes the damage from all attacks.
I felt that reading the Hosts aura could produce results about something not being right with the correct amount of successes, but I haven't really set any rules for that out atm.

2.) Essence Drain. Once attached the parasite spirit can drain the essence of the host. The normal rate is one week for one point of essence. The spirit may also make a test to increase the rate of essence drain, the spirit rolls a number of dice equal to it's force rating against a TN equal to the essence of the host, divide the time of one week by the number of successes.

Two points about this. First, the spirit doesn't have to roll, once attached, to drain essence only to increase the rate of drain. Also, the spirit does not have to actually drain the essence, if a particular Host is worth having around, they can not drain them for some time. Second, I tried to make it so that if a player actually got one of these attached to her/him they would have some time to get it off.

3.) Increaseing Force. A parasite spirit can increase it's force rating once it has drained a number of points of essence equal to twice it's current force rating.
I wasn't sure if this mechanic should work as written or if the essence drained should work to add to the spirits Spirit Power (or whatever that 'other stat' for spirits is).

That hits the main points of the spirits powers, but why should anyone ever let anything like this see the light of day? Well, of course they help out power-mad casters!

1.) Metamagic Abilities. The Host gains the metamagic abilities of Absorbtion and Siphoning without having to Initiate for them.
These abilities are actually under the control of the parasite and they let the Host use them...but an interesting point is that the spirit could use them even if the Host were disabled or otherwise distracted.

2.) Magical Aid. The parasite is also able to help the Host by using the Aid (or whatever the spirit ability is) power. This can make for very powerful villains though so use at GM discresion.

This has been a quick overview of the spirits and I envisioned more powerful spirits being able to control the Host, gain access to memories, etc.
I hope these are interesting and welcome any help in refining them. I have only played up to 3rd ed. and have never seen a 4th ed. book, so all my descriptions may be way off and for that I apologize.

lol guess the idea sucks. noone cares to comment at all

I really like the idea of a spirit masking and I think this type of spirit can be parsed out to make for a nice challenge for players.

The problem that hits me up front is tying the parasite spirit's force to its level of masking. To detect a masking character, it is an opposed roll between the masker (magic + intuition + initiate grade) and the perceiver (assensing + intuition).

A force 4 parasite spirit as you proposed, with force 4 being a fairly run of the mill power level, would have 12 dice (4 magic, 4 intuition, 4 initiate grade) to mask. If you put this against a *starting* mage (assume 4 assensing, 4 intuition) that would be 8 dice. The advantage definitely goes to the spirit. If you run a new mage specialized for perception, you are still dealing with maybe 13 dice (5 intuition, 6 assensing, +2 for specializing in reading auras). The advantage would only slightly go to such a specialized mage (who would be gimped quite a bit to handle this build). If you could imagine bumping up the spirit to even force 6, you are dealing with 18 dice vs whatever the mage can get.

I would propose instead a flat level 1 initiate power of masking. This would give a force 4 spirit 9 dice (4 magic, 4 intuition, 1 initiate grade) and scale up to a force 8 with 17 dice (8 magic, 8 initiative, 1 initiate grade). I think this would scale with character ability over time better and present a challenge but not an insurmountable one.

However, I really like this idea enough that I may do some parsing out of this for a campaign I am going to be starting soon!

Thanks for the feedback. I have been gone from the game for too long and haven't even seen a 4th ed. book so I was using what I remembered from 3rd (which was obviously not enough ) so I hadn't thought about how hard it would be for any char to actually break the Masking on the spirit.

A Tapeworm Spirit could be kinda nifty. You'd have the Celtic alp-luachra right there. Probably have a boosted Essence Drain from the other Parasites.

I could also see making a rather nifty alt-HMHVV from this idea.