Ok well I think you could implement this as a magic shop of some sort.

Well to start off with I know there is a spell or two that calls on spirits, but I was thinking of a store that really dealt with the spiritual plane. I was thinking that you could summon different spirits that actually did unique things. The spirits are under the control of whoever channeled them to this world and can only stay in the material plane for so long before they must go back. The spirits each have a special effect that could range from doing damage, stealing life, depleting energy, channeling damage from its master to itself, healing its master, channeling a certain element, etc.

Besides just channeling spirits you could channel spirit power into weapons and other things. It would be like an enchantment on your weapon that only lasted for a certain length of time. Depending on how good the enchantment was you could put a limit on it like once per week or once per topic etc. As for the effects they could again be an array of things from channeling elements into the weapon, ignoring armor, warding off foes, pointing you in the right direction, healing, etc.

Anywho that was just a bit of the info off the top of my head and you can tell me what you think. I was inspired by the guild wars ritualist and thought it would be something cool for Arda. So you can add stuff or tell me what you think.

Something similar has already been suggested here and is in the works.