Didn't really read the section on Invocation and Great Form spirits with a critical eye until now. I want to try to make sure I understand it right. The rules are divided between the Spirits section and the Metamagic section which makes it more difficult to comprehend. Also I want to poke holes in it because I'm seeing some exploitable things.
As I understand it, to summon a Great Form spirit you (obviously) need the Invocation metamagic. Here is the rules block in Invocation on pg 122:
Summoning a great form spirit is very similar to summoning a spirit normally, except as part of the same Major Action, the magician must invoke the spirit by making a Conjuring + Magic + Initiate Grade vs. (Force x 2) Opposed test; note that one of the dice added to the pool by the magician’s Initiate Grade is always a wild die. If the magician gets any net hits, those are added to the net hits from the summoning test to determine how many services the great form owes. If the magician does not get any net hits, the great form spirit goes berserk and attacks anything and everything within (Force x 10) meters for a number of combat rounds equal to the spirit’s net hits. Either way, the
magician must resist drain equal to the number of hits (not net hits) the spirit rolled. This is Stun damage unless it is higher than the character’s Magic rating, in which case it is Physical. If the drain is enough to knock the summoner unconscious (or dead), the spirit goes berserk for additional 1D6 combat rounds.
I take this to mean that summoning a great form is two tests wrapped up in one major action:
The Summoning Test
The Invocation Test
Only the Invocation test is detailed here, and it mentions a Drain Test. So it's actually:
The Summoning Test
The Summoning Drain Test
The Invocation Test
The Invocation Drain Test
(Two opposed tests and two simple tests just to do one action is not good design IMO)
It's unstated, but it seems logical to me that if the Summoning test fails, we don't go on to an Invocation Test. If the Summoning succeeds and the Invocation fails, the spirit always goes uncontrolled and berserk. If both succeed, the summoner's net hits from each test are added together to get the total services that the spirit owes.
To figure out what is great about a Great spirit, we need to go back to pg. 66. There appear to be zero mechanical differences between regular and Great spirits, except for access to a special selection of Powers. Unlike regular summoning, Great Form powers appear to depend on the number of hits gained by the summoner! That's according to the table on pg 66 titled "Great Form Spirit Hit Results". However, this table is not referenced by any of the text.
# OF HITS RESULT
4 Gain Magical Guard power (p. 69)
5 Gain an additional optional power (see powers listed by spirit type, pp. 147-8, SR6)
6 Gain greater spirit power appropriate to the spirit type
7 Astral Gateway power (p. 66)
8 Endowment power (p. 67)
9+ One additional power or increase Force by one with each hit beyond this amount
It's very beneficial then that Summoning and Invocation add to give us the total services the spirit owes. Since this is the final net hits #, I assume this is the number to use with the Great Form Spirit Hit Results table. No text references this table so I have to make that assumption. I also assume this table is cumulative. If I get 4 net hits the spirit has Magical Guard. If I get 8 net hits, the spirit has Endowment power and the powers from the lower results. I assume this because if the spirit ONLY gets the result of the net hits I got, this whole endeavor would feel extremely random. Also, if I got less than 4 net hits, my spirit is exactly the same as a regular spirit! Bad design.
Great Spirit powers have failures in other ways:
Divining power says it's available to Great Spirits but the only time it could be applied is on the 9+ hit result or GM fiat.
If you get 6 hits on a Kindred Spirit, you get Magical Guard, which you already would have received on 4 hits.
I'd recommend fixing both of these by making Divining the 6 hit result for Kin.
Exploits:
Because Powers are based on the number of net hits you get, the obvious exploit is to summon very small Force Great spirits. The fluff about Great spirits describes them as being bigger more powerful versions of regular spirits, but you summon them the same way. Pick a Force, summon against FX2, then an invocation test against FX2. Summoning against a F1 spirit gives you the best chance of getting access to powerful unique spirit powers like Astral Gateway. Astral Gateway doesn't care what Force the spirit is. Neither does Endowment. Exploiting this way is actually a really smart way to get a high Force great spirit. If you get 13 net hits against your Force 1 spirit (not unheard of, if net hits across both tests are combined, which I think they are), then you actually summoned a Force 6 great spirit for the price of a Force 1.
Endowment seems like another potentially exploitable power. I'm not sure how to interpret this text:
This power allows a spirit to give access to one of their powers to a target. The spirit retains the ability to use the power. A character is limited to only a single power at any given time.
There is no mention of limits for how the target may use the power endowed by the spirit. Two cases make the most sense though - either the target can use the power as often as they want, as limited by the description of the power, OR each use of the power by the target consumes one service of the great spirit. Since nothing is said, I assume they can use it as often as the power allows just like a regular critter.
It's also not clear to me whether powers granted in this way would use the spirit's stats for tests, or the target's stats. I can see arguments for both and I don't know where to land.
There's also some open questions I think. Could an astrally projecting character Materialize if they were granted that power by a spirit? Could it grant Sapience to a nonsapient target? Can a spirit endow a target with Endowment?
Reading this I feel like I opened up a can o' worms. Discuss!