This is the second draft of my spirit rules.
I find that I need to strip my posts down to the bare minimum and not drop these text walls. If you know Shadowrun, you know Mages can't summon Nature Spirits. I don't need to explain the game, just the converted mechanics.
Summoning a Spirit
To summon a spirit, the Magician states what type of spirits he is summoning, how many Hit Dice the summoned spirit will possess, and then makes a skill roll of Magic/Conjuring, modified by Charisma. Shamans might get a further bonus to the roll from Totems. The Magician takes a penalty to the roll equal to the difference between the spirit's Hit Die and his own. A Magician daft enough to conjure without at least Magic.Conjuring 0 is at -2.
On a success, the spirit appears immediately and owes the Magician one favor/service, plus one for each point by which the Magician made the roll. (So if his total skill roll is 11, he gets four favors: one for success and three for making the roll by three.) On a failure, nothing happens. A really mean GM might have a hostile spirit appear on the roll of snake eyes.
Either way, the Magician needs to resist Drain. This is a Mental Effect save, modified by Wisdom. The spirit's Hit Die are treated like a spell level, so a Magician has -2 to summon a spirit with 4-6 Hit Die, -4 to summon 7-9 Hit die, and so on. In addition, if the Magician summons a spirit with Hit Die greater than his own experience level, treat it as though he were overcasting a spell. (-1 for each spirit Hit Dire in excess of his experience level)
On a successful Drain Save, the Magician suffers no Drain. (Unless he was overcasting) Failure results in the same Drain as having failed a Drain Save when casting a spell. If the Magician is rendered unconscious by Drain, the spirit departs and owes him no favors.
Mages can bind a total number of elementals equal to three modified by Charisma.
Banishing a Spirit
Banishing a spirit is done through a series of banishing attacks. Each such attack consumes a Magician's combat round. The Magician must be able to see the spirit, which means he might have to switch to Astral perception. (Details on Astral mojo forthcoming)
To resolve the attempt, consult the Turn Undead table in D&D. Treat the Magician as a cleric of equivalent experience level and the spirit as an undead of equivalent Hit Dice. The Magician makes a Turn Undead check, though he can modify it by his Wisdom modifier and his levels in Magic/Conjure. (Trying to do with without the skill is at a -2)
On a success, the spirit loses a Hit Die and may not act this round if it has not already acted. If reduced to 0 Hit Dice, the spirit has lost the contest.
On a failure, the spirit may chose to immediately strike back. If it does so, the Magician temporary loses a level of Magic/Conjuring. If this would bring his skill below level 0, he temporarily loses a point of Wisdom modifier. If this would bring his Wisdom modifier below 0, the Magician has lost the contest.
A spirit that loses a banishment contest is immediately banished. A Magician who loses the contest loses consciousness for 1d4 rounds and cannot use any of his special abilities (spells, conjuring, Astral abilities) for 24 hours. The Magician regains lost Wisdom modifiers at the rate of one point per hour, and then lost levels of Magic/Conjuring at the rate of one point per hour.
Taking Control of a Spirit
If the spirit is "wild" or otherwise acting independent of a Magician's direction, treat this exactly like a banishment contest, but if the Magician wins, the spirit is immediately treated as if he summoned it. It owes him 1d3 favors, modified by Wisdom. (Not Charisma, because he has forced his will on the spirit rather than cajoled it) A Mage may choose to bind an elemental in the usual fashion and dismiss it for later service. After a contest of this nature, the Magician may allow the spirit to immediately regain up to its full Hit Dice.
If the spirit was summoned or is bound to another Magician, an extra step is required. First, resolve the contest as if it were a banishment contest. If the wresting Magician wins, he now makes an opposed roll against the enemy Magician. This is a contested skill roll between both Magicians' Magic/Conjuring skill, modified by Wisdom. The aggressor suffers a penalty equal to the difference of the defender's experience level and his own. (Yes, this is a bonus if the defending Magician is lower level.)
Example: A 4th level Mage trying to wrest an elemental from a 6th level Mage rolls at a -2 to skill. If an 8th level Mage tries to snatch an elemental from a 1st level Mage, he rolls at a huge +7.
If the aggressor wins, he gets control of the spirit, and it owes him 1d3+Wis mod favors, effective immediately. He can also restore the Hit Dice of the spirit. If the aggressor loses, he suffers the same result as failing to banish.